Some of the freshly trained al-Shabaab recruits were "burnt beyond recognition" and the death toll exceeds 75, one official said. Africa Command officials offered no other details except to say it did not kill or injure any civilians, suggesting the militants were in a camp or massing for an attack.
It was the largest United States air strike since a November 2017 attack, which killed about 100 al-Shabab fighters.
They are also authorized to conduct offensive airstrikes in the southern part of the country targeting al Shabab safe havens.
"This precision airstrike was the largest airstrike against al-Shabaab since November 21, 2017, when USA forces conducted an airstrike against an al-Shabaab camp killing approximately 100 terrorists".
Past year there were 31 such attacks and 15 in 2016, according to the Foundation for Defence of Democracies, a Washington-based research institute.
Somali Al-Shebab fighters as they gathered on February 13, 2012 in Elasha Biyaha.
The strike came after a recent spate of attacks that Al-Shabab has conducted against Somali security forces and their USA advisers across the country.
"We now assess this airstrike killed approximately 60 terrorists".
"A large concentration of Shabaab fighters a significant distance from its typical support zone in Jubba River Valley demonstrates the al-Qaeda branch's resilience across the country", wrote Bill Roggio and Alexandra Gutowski for the Foundation for Defence of Democracies' Long war Journal.
The US Africa Command spokesman said the air strike had no link to the anniversary of a massive bomb that killed almost 600 people in Mogadishu.
The strike took place on Friday in the vicinity of Harardhere, about 480km north-east of Mogadishu, Somalia's capital, the military said in a statement on Tuesday.
Since 2007, the United States military's combat drones have waged war against al-Qaeda's affiliates in Somalia, among them Al-Shabaab.