WASHINGTON -The United States dropped the “mother of all bombs” — the most powerful conventional bomb in the American arsenal — on a cave complex in Afghanistan on Thursday, the Pentagon said, unleashing a weapon so massive that it had to be dropped from the rear of a cargo plane.

The strike was the first combat use of what is formally named the GBU-43/B Massive Ordnance Air Blast. President Trump has bestowed additional authority on the Pentagon in his first months in office, which the military has argued will help it defeat Islamic State militants more speedily. Trump did not say whether he had personally approved Thursday’s mission. “What I do is I authorize my military,” Mr. Trump said after a meeting with emergency workers at the White House. He called the bombing “another very, very successful mission.”

While the damage from the bombing, which occurred at night in a remote area, was unclear, the strike quickly brought backlash. Hamid Karzai, Afghanistan’s former president, was among those who condemned it.

“This is not the war on terror but the inhuman and most brutal misuse of our country as testing ground for new and dangerous weapons,” Mr. Karzai wrote on Twitter. “It is upon us, Afghans, to stop the USA.”

As many as 36 suspected Isis militants were killed in Afghanistan when the US dropped "the mother of all bombs," its largest non-nuclear device ever unleashed in combat, the Afghan Defence Ministry said on Friday.

The claims have not been independently verified.

The Pentagon gave no casualty totals for the bombing, part of an intense air campaign against the militant group in Afghanistan. But in a separate announcement, the Pentagon said that an airstrike in Syria by the American-led coalition fighting the Islamic State there had killed 18 Syrian fighters allied with the United States, raising concerns about whether the White House is applying any rigour to the process of approving airstrikes in hot spots from Afghanistan to Syria.

The Syria strike — on Tuesday near the town of Tabqah, which Syrian fighters and American advisers are trying to capture — was the third American-led airstrike in a month that may have killed civilians or allies. Earlier bombing runs killed or wounded scores of civilians in a mosque complex in Syria and in a building in the west of Mosul, Iraq.

“We have the greatest military in the world,” Mr. Trump said. “We have given them total authorization, and that’s what they’re doing, and frankly, that’s why they’ve been so successful lately.”

American commanders in Iraq and Syria have been given more authority to call in strikes, a loosening of the reins that began in the last month of the Obama administration. But some national security experts said that Mr. Trump and the Pentagon risked inflaming anti-American sentiment in the Muslim world with their approach.

The number of civilian casualties reported in American-led strikes in Iraq and Syria has increased since Mr. Trump took office, and March was the deadliest month for civilians ever recorded by Airwars, a group that tracks bombings. Reports of civilian casualties in Iraq and Syria jumped to 3,471 from 1,782 the month before, the group said.