KABUL, Afghanistan — A day after the United States military dropped its most powerful conventional bomb on caves used by Islamic State affiliates in eastern Afghanistan, officials said on Friday that dozens of militants had been killed, but that they were still trying to assess the full extent of the damage. Residents said the blast had been felt tens of miles away.
The strike on Thursday targeted a set of mountain tunnels in the Achin district, a stronghold of the Islamic State’s regional affiliate, and it was the first use in combat of the GBU-43/B Massive Ordnance Air Blast, referred to as the “mother of all bombs.” The bombing was part of an intense air campaign against the Islamic State, with American airstrikes in Afghanistan averaging as many as 10 a day in the first two weeks of April.
Gen. Dawlat Waziri, a spokesman for the Afghan Defense Ministry, said initial information indicated that 36 militants had been killed and three large caves destroyed in the bombing in Nangarhar Province. However, Attaullah Khogyani, a spokesman for the provincial governor’s office, said 82 militants had been killed.
Gen. John W. Nicholson Jr., the commander of American and NATO forces in Afghanistan, said on Friday at a news conference in Kabul, the capital, that the Islamic State was using caves, tunnels and “an extensive belt of improvised explosive devices,” or roadside bombs, to resist Afghan and coalition operations.
The general said that he had been in constant touch with his chain of command, but that the decision to deploy such force was shaped by the battlefield realities and not by outside political factors. President Trump has given additional authority to military commanders since taking office, but he has not said whether he personally approved Thursday’s bombing mission.
“This is the first time we have encountered an extensive obstacle to our progress that was constituted by I.E.D.s, the presence of tunnels and caves, and therefore this was the appropriate weapon to use at this time,” General Nicholson said. “It was the right time to use it tactically against the right target on the battlefield, and it has enabled us to resume our offensive operations.”
Local officials in Achin said that Afghan commandos were advancing on the area and that smaller airstrikes had continued Friday morning.
“U.S. forces are providing air support and also support on the ground — there are some U.S. advisers with Afghan forces,” said Ismail Shinwari, the district governor of Achin.
Ahmad Jawid Salim, a spokesman for the Afghan Army commandos, said that the operation in Achin had been underway for 45 days but that progress had stalled in the Tangi Assadkhel area of the district, where the bomb was dropped. On Sunday, an American special forces soldier, Staff Sgt. Mark R. De Alencar, 37, was killed near there.
“Our foreign counterparts used all available weapons” to destroy the Islamic State havens in Tangi Assadkhel, Mr. Salim said. “But because the posts and havens of I.S. were very strong, it was decided to use this big bomb.”
Mr. Salim said that the Afghan commandos were given notice that the bomb would be used and that they pulled back about two miles before it was dropped. “We were getting reports minute by minute, and we were aware that the bomb would drop in 30 and then 20 seconds,” he said.
There were no initial reports of civilian casualties from the explosion, and Afghan and American military officials said that precautions had been taken to avoid harming noncombatants. Most civilians in Achin have been displaced since 2015, when the Islamic State turned the district into a stronghold.
Mr. Salim said that there had been only one civilian family in Tangi Assadkhel, and that it had been evacuated before the strike.
But Malik Kamin, a tribal elder from the nearby area of Shadal Bazaar, said some civilians had been in Tangi Assadkhel when the bomb fell. He said commandos who went in after the bomb was dropped had found two disabled women and an elderly man there and brought them to Shadal Bazaar.
General Nicholson said there were no indications that civilians had been wounded or killed. There was “surveillance over the area before, during and after the operation, and now we have Afghan and U.S. forces on the site and see no evidence of civilian casualties,” he said at the news conference.
The bomb, which was dropped from a cargo plane, weighed about 10 tons, and its force was felt across Achin and even in neighboring districts.
One tribal elder who lives less than two miles from Tangi Assadkhel said the blast was so strong that residents of his village thought that it had been the target. Shrapnel and rocks as heavy as five pounds fell on his house, he said. A resident of the nearby Pekhe area said four houses there, about three miles from the blast site, had been destroyed.
Both residents spoke on the condition of anonymity, fearing retaliation if the Islamic State found its way back to their area.
Haji Ghalib, governor of Bati Kot, a district about 20 miles from Achin, said he had felt the blast there. “People in other districts also felt it,” he said. “From check posts in Achin some guys called me, and they were asking, ‘What was that?’ It was very big — for a moment, big flames were rising from the mountain, the whole area was bright.”
The Islamic State’s regional affiliate in Afghanistan, largely made of former members of the Pakistani Taliban, was rapidly expanding in eastern Afghanistan during much of 2015 and 2016. In March 2016, American military officials estimated that the group had 2,000 to 3,000 fighters across 11 districts.
After multiple operations and extensive airstrikes, that number has been reduced to about 700 fighters across three districts, officials say. The efforts involved several ground operations by Afghan soldiers and commandos advised by American military special forces. But they were also accompanied by an intense air campaign that included B-52 bombers last year, staples of the early part of the war that had not been used for many years.
The Islamic State has claimed responsibility for several suicide bombings in urban centers, most recently at the gates of the presidential palace in Kabul on Wednesday, an attack that killed at least five people.
In the past, urban attacks were often the work of the Haqqani network, a brutal arm of the Taliban. In a sign of the complexity of the war, Afghan and American officials have expressed concern that there might be an overlap in the enabling networks used by the Haqqani group and the Islamic State for bombings in cities.
“One of the things we are concerned about, and the reason we think the entire world needs to be focused on Afghanistan, is the potential for convergence among the various terrorist groups in this area,” General Nicholson said.
Some of the tunnels and caves in the complex bombed on Thursday dated from the fight against the British Empire, said Mr. Ghalib, the Bati Kot district governor, who was Achin’s governor for years. More tunnels were added during the Soviet occupation of Afghanistan, and still more by the Islamic State.