(CNN) -- Claims and counterclaims came thick and fast Friday in response to the White House's declaration hours earlier that it believes the Syrian government has crossed a "red line" in using chemical weapons against rebels.
That conclusion -- declared for the first time Thursday -- is prompting the United States to increase the "scale and scope" of its support for the opposition, the White House said, although officials stopped short of saying it will put weapons in the hands of rebels.
The U.S. report won backing from the British government Friday -- but Syria and its allies in Moscow quickly sought to cast its integrity into doubt.
The Syrian foreign ministry accused Washington of releasing "a statement full of lies regarding the use of chemical weapons in Syria," according to a statement on state TV.
And a government statement reported by state news agency SANA accused the United States of using "flagrant tricks to come up with any possible mean to justify the decision of President Barack Obama to arm the Syrian opposition."
Washington is "clearly exercising scandalous double standards in dealing with terrorism," the statement said. The Syrian government under President Bashar al-Assad habitually refers to the rebels as terrorists.
An Assad loyalist who spoke to CNN's Frederik Pleitgen in Damascus said he believes the United States is "inventing stories" about the government's use of chemical weapons "because our army is winning."
A boost in support by the United States for the rebels could put at risk the gains made by Syrian forces in recent days, especially in central and northern Syria, with the help of Hezbollah fighters from Iran.
But many questions remain over what form the U.S. support could take and whether it would include the provision of small arms and ammunition, or heavy arms such as anti-aircraft weapons.
U.S. Sen. John McCain, who has repeatedly called on the Obama administration to step up its support of the rebels, told CNN's Situation Room on Thursday that the rebels need anti-tank and anti-aircraft weapons.
"They need a lot more military assistance," McCain, an Arizona Republican, said, adding that the United States and its allies also need to "establish a 'no-fly' zone to create a safe area" within Syria.
"You can't do it with half measures. You can't do it with just supplying weapons," he said.
Russia: Evidence is unconvincing
Meanwhile, an aide to Russian President Vladimir Putin said Friday that Moscow is unconvinced that the Syrian government used chemical weapons against rebels in the country's civil war, according to Russia's state-run news agency RIA Novosti.
Presidential aide Yuri Ushakov told reporters in Moscow that the United States had shown Russian officials data and information on this score but that it was "unconvincing," according to the news agency.
"The Americans tried to show us information about the Assad's regime using chemical weapons. But if I have to be direct, what we saw does not look convincing to us," Ushakov is quoted as saying.
A senior Russian lawmaker also dismissed the U.S. report as a fabrication.
"Information about the usage of chemical weapons by Assad is fabricated in the same way as the lie about [Saddam] Hussein's weapons of mass destruction [in Iraq]," Alexei Pushkov, head of the Russian lower house of parliament's international affairs committee, said on Twitter, according to RIA Novosti.
Obama "is going the same way" as former President George W. Bush did then, Pushkov said, alluding to the claims made against Iraq that preceded the U.S.-led invasion in 2003.
However, UK Foreign Secretary William Hague backed the U.S. government's assessment and called for a coordinated response from the international community.
"We agree with the US assessment that chemical weapons, including sarin, have been used in Syria by the Assad regime," he said, according to a UK Foreign Office statement.
"The United Kingdom has presented evidence of the use of chemical weapons in Syria to the U.N. investigation, and we have been working with our allies to get more and better information about the situation on the ground. We condemn the deplorable failure of the Assad regime to cooperate with the investigative mission."
Syria has long maintained that rebels, not government forces, are behind the use of chemical weapons. It also went to the United Nations with its claims, but al-Assad would not allow U.N. inspectors into the country to try to verify the claims.
Earlier Friday, NATO Secretary General Anders Fogh Rasmussen said the international community had made clear that any use of chemical weapons is "completely unacceptable and a clear breach of international law."
Speaking in Brussels, Belgium, he said he welcomed "the clear U.S. statement" on Syria's alleged chemical weapons use and that it was a matter of great concern.
"It is urgent that the Syrian regime should grant access to the United Nations to investigate all reports of chemical weapons use," he said.
"As for NATO, the Patriot deployment will ensure effective protection for Turkey against any missile attack, whether the missiles carry chemical weapons or not," he said.
Rasmussen said that he still believed "the right way forward is a political solution" and that he embraced the joint U.S.-Russian initiative to call an international conference on Syria.
"I urge all parties involved, the government and the opposition in Syria, to attend the conference and hopefully pave the way for a long-term, sustainable political solution," he said.
He also called for an immediate end to the bloodshed in Syria.
The United Nations said Thursday there have been more than 92,000 documented deaths in Syria since March 2011, when a brutal government crackdown on Arab Spring-inspired protests devolved into an armed conflict.
Crossing 'clear red lines'
Details of the U.S. government report into chemical weapons use were given by Ben Rhodes, the deputy national security adviser for strategic communications, in a statement released by the White House.
"The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date; however, casualty data is likely incomplete," he said.
"While the lethality of these attacks make up only a small portion of the catastrophic loss of life in Syria, which now stands at more than 90,000 deaths, the use of chemical weapons violates international norms and crosses clear red lines that have existed within the international community for decades," Rhodes added.
Rhodes later told reporters on a conference call that the president has made a decision about military support for the rebels but stopped short of saying the U.S. government would put weapons in the hands of rebels.
The president has previously said he did not foresee a scenario with "American boots on the ground in Syria."
Rhodes also said no decision has been made by Obama over whether to institute a no-fly zone in Syria, something rebel forces have said is needed to halt al-Assad's aerial bombardment of their strongholds.
The administration plans to share its findings with Congress and its allies, and it will make a decision about how to proceed "on our own timeline," Rhodes said.
Syria will be among the chief topics for Obama at the Group of Eight summit in Northern Ireland next week, where Rhodes said the president will share the U.S. findings on al-Assad's use of chemical weapons.
The administration says it believes that al-Assad's government maintains control of the chemical weapons and that there is "no reliable, corroborated reporting to indicate that the opposition in Syria has acquired or used chemical weapons," Rhodes' statement said.
Rhodes gave no indication of how many times al-Assad's forces used chemical weapons, but a U.S. Senate source briefed on the matter said the administration believes Syria used such weapons on at least eight occasions.
The White House announcement came at a critical time for the Syrian opposition, which has suffered a series of significant losses in recent weeks.
The setbacks in large part have coincided with the arrival of thousands of Hezbollah Shiite fighters, backed by Lebanon and Iran, to reinforce al-Assad's forces battling the mainly Sunni uprising.
After months of gaining ground, the rebels this month lost Qusayr -- one of its strongholds near the Lebanese border -- which was considered essential for its supply route.
Until now, the United States has limited its aid to rebels, providing communications equipment, medical supplies and food.
What complicates any U.S. military support for the opposition is that many of the rebel fighters are militants with pro-al Qaeda sympathies, the same stripe of militants America has battled in Iraq and Afghanistan.
They include the al-Nusra Front, a rebel group that the United States says has links to al Qaeda.
As recently as last week, France's foreign minister said sarin gas had been used several times in the Syrian civil war, citing results from test samples in France's possession.
In early May, the head of the U.N. Independent International Commission of Inquiry on Syria said that evidence points to the use of sarin by Syrian rebel forces. But the commission later issued a news release saying it "has not reached conclusive findings as to the use of chemical weapons in Syria by any parties to the conflict."
In April, the head of the Israeli military's intelligence research said the Syrian government is using chemical weapons against rebel forces.
Sarin gas can be hard to detect because it is colorless, odorless and tasteless. It can cause severe injuries -- including blurred vision, convulsions, paralysis and death -- to those exposed to it.
Analysts believe the Syrian government may have one of the largest stockpiles of chemical weapons in the world. Specifically, the supply is believed to include sarin, mustard and VX gases, which are banned under international law. Syria has denied the allegation.
The Chemical Weapons Convention prohibits the production, stockpiling and use of chemical and biological weapons. Syria is not one of the 188 signatories to the convention.
In recent months, reports have repeatedly surfaced that Syrian forces have moved some of the chemical weapons inventories, possibly because of deteriorating security in the country, raising fears the stockpile could fall into the hands of al Qaeda-linked groups working with the opposition should al-Assad's government fall.
As a result, the United States has been talking with neighboring countries about the steps needed to secure the weapons should al-Assad be forced from office.
CNN's Barbara Starr, Jessica Yellin and Chelsea J. Carter contributed to this report.