WASHINGTON — American warplanes struck a training camp in Somalia belonging to the Islamist militant group Al Shabab, the Pentagon said Monday, killing about 150 fighters who officials said were preparing an attack against US troops and their regional allies in East Africa.

The strikes occurred Saturday at a training facility called Camp Raso, about 120 miles north of Mogadishu, and came as Al Shabab fighters were nearing the end of “training for a large-scale attack’’ on forces belonging to the African Union in Somalia, officials said.

They were bombed during what US officials said they believed was a graduation ceremony, and the warplanes dropped a number of precision-guided bombs and missiles on them.

“They were standing outdoors in formation,’’ one official said.

The camp was destroyed, according to Navy Captain Jeff Davis, a Pentagon spokesman. He added that the military believes there were no civilian casualties, the Associated Press reported.

Davis said the United States estimated that as many as 200 fighters had been at the camp, including a number of trainers.

In a separate development Monday, the White House announced that it will disclose how many people have been killed by American drones and other counterterrorism strikes since 2009, when President Obama took office, the AP reported.

Lisa Monaco, Obama’s counterterrorism and homeland security adviser, said the report will be released ‘‘in the coming weeks,’’ casting it as part of a commitment to transparency for US actions overseas.

The list will not include major war zones such as Iraq, Syria, and Afghanistan, but will focus on strikes against extremist targets in other regions such as Pakistan, Libya, Yemen, Somalia, and other locations in North Africa.

Monaco said the figures would be disclosed annually in the future, although it will ultimately be up to Obama’s successor to decide whether to continue the practice

The United States has a number of Special Operations forces in Somalia, and Defense Department officials said they were also believed to have been targets of the planned attacks.

Al Shabab fighters killed in the strikes were “nearing the completion of the end of their training,’’ Davis said. He said the strikes “will degrade Al Shabab’s ability’’ to attack its neighbors and the West.

The latest strikes come as East Africa analysts say that Al Shabab, the group responsible for the 2013 attack on the Westgate mall in Kenya, is making a comeback after US strikes killed the group’s top leadership in 2014. Last month, Al Shabab claimed responsibility for an attack on a popular hotel and a public garden in Mogadishu that killed 10 people and injured more than 25 others.

In the past two months, Al Shabab militants have killed more than 150 people, including Kenyan soldiers stationed at a remote desert outpost and beachcombers in Mogadishu. In addition, the group has claimed responsibility for a bomb placed aboard a Somali jetliner that tore a hole through the fuselage.

Pentagon officials would not say how they knew that Al Shabab fighters killed on Saturday were training for an attack on US and African Union forces, but the militant group is believed to be under heavy US surveillance.

Some experts say that Al Shabab, an Al Qaeda affiliate, is in a competition with the Islamic State to show that it has not been eclipsed.

The United States doesn’t publicly disclose all the places its drones operate, so the new Pentagon casualties report isn’t expected to detail specific countries where people died.

Instead, it will offer an aggregate assessment of casualties outside of areas of ‘‘active hostilities’’ — a designation that takes into account the scope and intensity of fighting and is used to determine when Obama’s specific counterterrorism policies apply.

Iraq and Syria, where US airstrikes are pummeling the Islamic State group, currently are on that list and won’t be in the report.

‘‘There will obviously be some limitations on where we can be transparent, given a variety of sensitivities — including diplomatic,’’ said White House spokesman Josh Earnest.

Obama’s move to shed more light on the drone wars comes as the United States struggles to contain extremist groups and violent ideologies that are growing and spreading.

For example, the Islamic State militant group, which the US-led coalition is fighting in Iraq and Syria, is spreading to undergoverned places in Libya and Afghanistan, and is spawning affiliates and recruits around the world.

Monaco, the counterterrorism adviser, described the strikes as one tool in a fight against terrorism that has entered a new, unpredictable phase nearly 15 years after the 9/11 attacks. In place of top-down, well-organized groups like Al Qaeda, the threat has shifted to a diffuse array of smaller groups and lone actors in what Monaco dubbed ‘‘do-it-yourself terrorism.’’

In central Somalia on Monday, a bomb exploded in a piece of luggage at an airport in a center of the country, wounding three people, a police official said.

The bomb went off at a checkpoint as soldiers searched through bags before passengers were allowed to board, said Ahmed Nur, a police official in the town of Beledweyne, where the incident happened. An African Union peacekeeper was among the wounded, he said.

Al Shabab, claimed responsibility for the blast.

Last month a bomb exploded aboard a commercial passenger jet, creating a hole in the fuselage, blowing out the suspected bomber and forcing the plane to make an emergency landing in Mogadishu. Al Shabab claimed responsibility for that attack.