WASHINGTON — American aircraft on Saturday struck a training camp in Somalia belonging to the Islamist militant group the Shabab, the Pentagon said, killing about 150 fighters who were assembled for what American officials believe was a graduation ceremony and prelude to an imminent attack against American troops and their allies in East Africa.

Defense officials said the strike was carried out by drones and American aircraft, which dropped a number of precision-guided bombs and missiles on the field where the fighters were gathered. Pentagon officials said they did not believe there were any civilian casualties, but there was no independent way to verify the claim. They said they delayed announcing the strike until they could assess the outcome.

It was the deadliest attack on the Shabab in the more than decade-long American campaign against the group, an affiliate of Al Qaeda, and a sharp deviation from previous American strikes, which have concentrated on the group’s leaders, not on its foot soldiers.

It comes in response to new concerns that the group, which was responsible for one of the deadliest terrorist attacks on African soil when it struck a popular mall in Nairobi in 2013, is in the midst of a resurgence after losing much of the territory it once held and many of its fighters in the last several years. The planned attack on American and African Union troops in Somalia, American officials say, may have been an attempt by the Shabab to carry out the same kind of high-impact act of terrorism as the one in Nairobi.