The US has launched an attack against a "known al-Qaeda terrorist" in southern Somalia, the Pentagon says.
Islamist leader Hassan Turki is wanted by the US
Three missiles hit Dhoble town early on Monday, reportedly killing four people and wounding 20.
People are fleeing the town, fearing more strikes. Residents say planes could still be seen flying overhead on Monday morning.
Islamist insurgents seized the town last week and reports said a leader, Hassan Turki, had been in the area.
Mr Turki is on the US list of "financers of terrorism".
US Defense Department spokesman Bryan Whitman refused to give the identity of the target, whether the strike had achieved its goal or how the strike had been carried out.
A US military official, who refused to be named, told the AFP news agency that at least one cruise missile had been fired.
Meanwhile, Islamists have attacked the town of Bur Hakaba, leaving the local police chief and four others dead.
The BBC's Mohammed Olad Hassan in Mogadishu says the Islamists have adopted a new strategy of launching attacks outside the capital.
Dhoble resident Fatuma Abdullahi told the BBC they were woken up by "a loud and big bang".
"When we came out we found our neighbour's house completely obliterated as if no house existed here," he said.
Another resident said: "Right now - in full daylight - the planes keep flying over us. They are so low that we're deafened by their engines."
"We are poor civilians living in a simple town - what have we done to deserve this bombing?"
Local official Ali Hussein told the BBC that many people were fleeing the town.
The border with Kenya has been closed for the past year.
Islamist spokesman Sheikh Mukhtar Robow said the US was trying to hit Islamist hideouts in the area.
"The Americans bombed the town and hit civilian targets, thinking that they were Islamist hideouts. They used an AC-130 plane," he told the AFP news agency.
The US bombed the area a year ago and residents said the same plane was again involved.
There have been reports that the Islamists have been regrouping in the area around Dhoble in recent weeks.
Somalia is the world's worst place for children, the UN says
They were ousted from the capital, Mogadishu in December 2006 by government forces, backed up by Ethiopia, with some intelligence from the US.
Dhoble was the last town they held.
The US has an anti-terror task force based in neighbouring Djibouti.
The US accused the Somali Islamists of harbouring those responsible for the 1998 attacks on its embassies in Kenya and Tanzania.
The Islamists denied this, as well as reports they had links to al-Qaeda.
Somalia has not had an effective national government since 1991.
Last month, a senior UN official told the BBC that Somalia was the worst place in the world for children.