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

John Porter's brother Mike, 66, died at the scene after the men were knocked to the ground by cows in Wiltshire.
After an inquest jury returned a verdict of accidental death, he said he hoped more could be done to prevent a further tragedy.
The jury noted that there had been previous attacks in the same fields.
And jurors noted a lack of separation between cattle and public, in a narrative accompanying their verdict.
Mr Porter, 74, from Monkton Combe, near Bath, told the BBC: "Accidents will always happen, sadly. But I do think that we do need to look at ways in which we can make our footpaths safer."
His brother was visiting from Edinburgh in May 2013 when they walked dogs on leads across a public footpath through Elbow Field, Turleigh. They were knocked to the ground by two cows and stamped on. Mike died at the scene of internal bleeding.
The inquest heard that Simon Dark, Bleddyn Griffiths and David Billington had also suffered injuries after being trampled in the fields in 2008 and 2011 - although it was not possible to say whether the same cows were involved.
The farmer Brian Godwin, 81, had been visited by the Health and Safety Executive after Mr Billington was injured. He had put in place some safety measures, including signs and some electric fencing. He has since sold the cattle and is trying to establish a new herd.
The lawyers representing Mike Porter's partner Adrienne Sillar and their two sons said there was a "clear issue" about whether Elbow Field should remain a public right of way and "whether cows should share the enclosure".
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.
[NEXT_CONCEPT]
A man who survived a cow trampling attack in which his brother was killed has told the BBC public footpaths need to be made safer.