The militants do not meet in such large numbers in "one spot" because of the threat of an attack, a spokesman said.
He did not give details of casualties.
A resident said that al-Shabab had arrested several people from the remote central Somali village of Raso, after the US bombed its training camp in a nearby forested area on Saturday.
The US said the strike,  by both drones and manned aircraft, was aimed at preventing a "large-scale attack" by the al-Qaeda-linked militants.
The resident told the BBC Somali service that al-Shabab recruits from around Somalia had been converging on the camp, about 195km (120 miles) north of the capital, Mogadishu, since last week.
The US strike appeared to have led to heavy casualties, with al-Shabab reinforcements arriving to collect the dead, the resident added.
Fighters who survived the assault had now left the area, he said.
A spokesman for al-Shabab, Sheikh Abdiasis Abu Musab, confirmed to Reuters news agency that the US had bombed an area controlled by the militants.
However, the US had exaggerated the number of casualties, he said.
"We never gather 100 fighters in one spot for security reasons. We know the sky is full of planes," the spokesman added.
On Tuesday, Pentagon spokesman Capt Jeff Davis said that initial assessments were that more than 150 fighters had been killed at the training facility.
The camp had been under surveillance for some time, Capt Davis said.
"We know they were going to be departing the camp and they posed an imminent threat to US and [African Union] forces," he added.
A Somali official said their intelligence service cooperated with the US ahead of the air strikes, the Associated Press news agency reports.
The US has a huge military base in neighbouring Djibouti.
In a 2008 strike, it killed al-Shabab leader Adan Hashi Ayro. His successor Ahmed Godane was killed in a 2014 strike.
28 February - on a restaurant and busy junction in Baidoa killing at least 30 people
26 February - on Mogadishu's SYL hotel killing nine people
21 January - on a restaurant at Mogadishu's Lido beach killing 20 people
15 January - on a Kenyan base in el-Ade that Somalia's president said killed at least 180 soldiers
Militant Islamist group al-Shabab has dismissed as exaggerated US claims that more than 150 of its fighters were killed in an air strike in Somalia.