WASHINGTON — The United States dropped the “mother of all bombs” — the most powerful conventional bomb in the American arsenal — on an Islamic State 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 the Islamic State more speedily. Mr. 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.”

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 rigor to the process of approving airstrikes in hot spots from Afghanistan to Syria.