BEIRUT — Al-Qaida's leader has tried to end squabbling between the terror network's Syrian and Iraqi branches, ordering the two groups to remain separate after an attempted merger prompted a leadership dispute between them.

This came as Syrian rebels battled Monday in a renewed push to capture a government air base in the north, while the regime was said to be preparing for a major offensive to retake opposition-held areas in the province of Aleppo.

The Qatar-based Al-Jazeera TV reported that al-Qaida leader Ayman al-Zawahri urged leaders of the Iraqi al-Qaida branch and the Nusra Front in Syria to end their disagreements and "stop any verbal or actual attacks against one another."

The TV said al-Zawahri's call came in a letter sent to the station and posted on its website late Sunday. The letter's authenticity could not be independently verified. The Britain-based Syrian Observatory for Human Rights, which has a network of activists on the ground in Syria, said it also acquired a copy of the letter but did not provide other details.

Al-Zawahri's call could also reflect a bid to carve out a more significant role for al-Qaida in the Syria civil war. Jabhat al-Nusra, or the Nusra Front, is the most powerful rebel force fighting to topple President Bashar Assad.

In April, al-Qaida in Iraq said it had joined forces with the Nusra Front, forming a new alliance called the Islamic State in Iraq and the Levant.

Hours after the announcement, Nusra Front leader Abu Mohammad al-Golani appeared to distance himself from the merger, saying he was not consulted. Instead, he pledged allegiance to al-Zawahiri.

In Sunday's letter, al-Zawahri chastises the head of al-Qaida in Iraq, Abu Bakr al-Baghdadi, saying he announced the merger without consulting al-Qaida's leadership. He also admonished al-Golani for publicly distancing himself from the merger.

"The Islamic State in Iraq and the Levant will be abolished," al-Zawahri said, adding that Nusra Front will remain an independent branch of al-Qaida. Al-Baghdadi and al-Golani are to stay on as leaders of their respective branches for another year, after which the al-Qaida leadership will decide whether they will keep their posts or be replaced.

Assad's government in April seized upon the reported merger to back its assertion that it isn't facing a true popular uprising but a foreign-backed terrorist plot.

The merger had also caused friction among rebels on the battlefield who feared the announcement would further discourage Western powers discussing funneling weapons, training and aid toward rebel groups and army defectors.

On Monday, rebel forces advanced inside the sprawling air base of Mannagh near the border with Turkey, activists said. The Observatory said rebels captured a building inside the base, which has been under siege for months. The opposition's Aleppo Media Center said rebels destroyed several army vehicles and captured the observation tower.

Activists also reported clashes around the predominantly Shiite villages of Nubul and Zahra, besieged by rebels for a year. Aleppo-based activist Mohammed al-Khatib said military reinforcements, including Hezbollah fighters, have been sent to parts of Aleppo, including the two Shiite villages and north-western parts of the city. He said the government was using helicopters to reinforce its positions and resupply in those areas.

The Shiite military group has openly joined the fight in Syria and was key in assisting regime forces in recapturing the strategic town of Qusair last week.

Syrian state-run media and the Hezbollah-owned Al-Manar TV have said the regime is preparing an offensive reportedly named Operation Northern Storm to recapture Aleppo.

Moved by the Assad regime's rapid military advance, the Obama administration began discussing Monday whether to approve lethal aid for the beleaguered rebels, and U.S. officials said a decision could come later this week.

In Washington, State Department spokeswoman Jen Psaki said the intense preparation for a siege on Aleppo "reaffirms the urgent need for the international community to focus its efforts on doing all we can do to support the opposition as it works to change the balance on the ground."

Opposition leaders have warned Washington that their rebellion could face devastating and irreversible losses without greater support.

Also Monday, a roadside bomb lightly damaged a van that was heading from Lebanon to Syria, Lebanese security officials said. The van was hit by the bomb, detonated remotely, in the eastern Bekaa valley but kept driving toward the border, crossing into Syria, said the officials, speaking on condition of anonymity in line with regulations. There appeared to be no casualties in the bombing.

Syria's conflict started with largely peaceful protests against Assad's regime in March 2011 but eventually turned into a civil war that has killed more than 80,000 people, according to the United Nations.

Lebanon is bitterly divided over the war next door, with gunmen from rival religious sects fighting on opposite sides of the conflict. Lebanese Sunnis mostly back the opposition while many Shiites in Lebanon support Assad. The Syrian regime is dominated by members of the president's minority Alawite sect, an offshoot of Shiite Islam.

Lebanese Shiite Hezbollah forces have taken an increasingly prominent role in Syria's fighting and were key in helping Assad's troops capture the strategic town of Qusair near the border with Lebanon, following weeks of battles with rebels.

On Monday, Syria's Defense Minister Gen. Fahd Jassem al-Freij said Qusair's capture last week was a "main point toward restoring security and stability to every inch of our nation."

In apparent retaliation by the rebel side, scores of rockets have been fired from Syria into Hezbollah strongholds in northeastern Lebanon.

Senior Hezbollah official Sheik Nabil Kaouk said it will not change it position on Syria, regardless of "how much local, regional and international pressure increases" on the Lebanese group.

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Associated Press writer Bradley Klapper contributed to this report from Washington.

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  • Turkey

    Turkey has struck the Syrian military repeatedly in response to shelling and mortar rounds from Syria since Oct. 3, when shells from Syria struck the Turkish village of Akcakale, killing two women and three children. The incident prompted NATO to convene an emergency meeting and Turkey sent tanks and anti-aircraft batteries to the area. Turkey's military has also scrambled fighter jets after Syrian helicopters flew close to the border. <em>Caption: Turkish soldiers patrols as Syrian nationals pass the border between Syria and Turkey on November 10, 2012, near the town of Ceylanpinar. (BULENT KILIC/AFP/Getty Images)</em>

  • Turkey

    There are about 120,000 Syrian refugees sheltering in Turkish camps, with up to 70,000 more living in Turkey outside the camps. Thousands more wait at the border, held up as Turkey struggles to cope with the influx. Turkey also hosts much of the opposition and rebel leadership. <em>Caption: A Syrian-Kurdish woman refugee sits in the courtyard of a house in the Turkish town of Ceylanpinar, bordering Syria, on November 10, 2012. (PHILIPPE DESMAZES/AFP/Getty Images)</em>

  • Turkey

    Turkey has called for a buffer zone in Syria where the opposition and civilians would be protected, a step that would likely require international enforcement of a no-fly zone. Russia and China have blocked robust moves against the Syrian regime at the U.N. Security Council, and the United States has been reluctant to use its military in another Mideast conflict. <em>Caption: Turkish soldiers patrols as Syrian nationals pass the border between Syria and Turkey on November 10, 2012, near the town of Ceylanpinar. (BULENT KILIC/AFP/Getty Images)</em>

  • Israel

    Israel on Monday became the second country to strike the Syrian military, after Turkey. An Israeli tank hit a Syrian armored vehicle after shells from fighting in Syria exploded in Israel-controlled Golan Heights. A day earlier, Israel fired a warning shot near a group of Syrian fighters. <em>Caption: Israeli tanks, one in position, the other getting into a firing position in the Israeli-controlled Golan Heights overlooking the Syrian village of Bariqa, Monday, Nov. 12, 2012. (AP Photo/Ariel Schalit)</em>

  • Israel

    Syrian shells have exploded inside the Golan several times in recent weeks damaging apple orchards, sparking fires and spreading panic but causing no injuries. In early November, three Syrian tanks entered the Golan demilitarized zone, and in a separate incident an Israeli patrol vehicle was peppered with bullets fired from Syria; no one was hurt in the incident and the Israeli military deemed it accidental. <em>Caption: Smoke rises after shells fired by the Syrian army explode in the Syrian village of Bariqa, Monday, Nov. 12, 2012. (AP Photo/Ariel Schalit)</em>

  • Israel

    There is concern in Israel that Assad may try to spark a conflict with Israel, opening up the potential for attacks by Lebanon's militant Hezbollah and Hamas in the Gaza Strip. Israel has also warned that Syria's chemical weapons could be turned on the Jewish state. Still, while no friend of Assad, Israel is also worried that if he is toppled, Syria could fall into the hands of Islamic extremists or descend into sectarian warfare. <em>Caption: Israeli troops and UN peacekeepers inspect on November 8, 2012 the area where three mortar shells fired from Syria landed in Alonei Habashan in the Israeli-occupied Golan Heights, which Israel seized from Syria in 1967. (JALAA MAREY/AFP/Getty Images)</em>

  • Lebanon

    Mortars and shells from the Syrian side regularly crash in Lebanon, causing several casualties, though Lebanese forces have never fired back. More dangerously, Syria's conflict has heightened deep rivalries and sectarian tensions in its smaller neighbor. Lebanon is divided between pro-Assad and anti-Assad factions, a legacy of the nearly three decades when Damascus all but ruled Lebanon, until 2005. Assad's ally, the Hezbollah militia is Lebanon's strongest political and military movement. <em>Caption: Lebanese army commandos deploy in the Bab al-Tabbaneh and Jabal Mohsen neighbourhoods where clashes are taking place between Sunnis and Alawites in the northern city of Tripoli on October 23, 2012. (JOSEPH EID/AFP/Getty Images)</em>

  • Lebanon

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  • Lebanon

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  • Jordan

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  • Jordan

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  • Iraq

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  • Iraq

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