The Pentagon initially characterized the attack as a drone strike, but later clarified that the operation involved both manned and unmanned aircraft.
The strike took place Saturday, 120 miles north of Mogadishu at a training site called "Raso Camp," according to Pentagon spokesman Capt. Jeff Davis.
However, Al-Shabaab denied the Pentagon report in in a short statement posted on the group's Twitter account.
"Harakat Al Shabaab Al Mujahideen denies the claims of killing 150 fighters in recent airstrike, as was reported by the Pentagon," the group said.
'Hasten their annihilation'
A spokesman for Somali Prime Minister, Omar Abdirashid Ali Sharmarke, told CNN the strike was part of a joint effort against Al-Shabaab.
"We welcome the precision drone attack on the Shabaab base in collaboration with the work of the SNA -- Somali National Army," spokesman Abdisalam Aato said.
"The attack will severely minimize the threat of Shabaab. This attack will boost the ground attack on Shabaab and hasten their annihilation."
Readying for 'large-scale attack'
Davis said the camp had been under observation by U.S. Special Operations forces for several weeks. The camp had about 200 fighters on site including Al-Shabaab trainers.
Separately, a U.S. official told CNN that the Al-Shabaab fighters were standing outside in military formation when the strike occurred.
Intelligence indicated the group was training for some time and was in the final stages of getting ready to conduct a "large-scale attack," Davis said.
Pentagon spokesman Peter Cook said in a statement that the attack was taken in defense of "our African Union Mission" in Somalia and that "The fighters who were scheduled to depart the camp posed an imminent threat" to the mission.
He added, "The removal of these fighters degrades Al-Shabaab's ability to meet the group's objectives in Somalia, including recruiting new members, establishing bases, and planning attacks" on U.S. and African Union forces.
The military did not specify the location of where the U.S. thought an attack might happen, but it is well known that the U.S. maintains a limited military presence at the airport in Mogadishu and that small numbers of U.S. Special Operations forces have traveled to other locations in the country.