An air strike in al-Shabab-held southern Somalia has killed four people, including three foreigners, eyewitnesses have told the BBC.
The strike destroyed two vehicles in a convoy in an area known as K60 because it is around 60km (35 miles) south of the capital, Mogadishu, they say.
It is not clear who fired the missiles, reportedly shot from five helicopters.
Al-Shabab recently announced it was joining al-Qaeda and is said to have some 200 foreign fighters.
The BBC's Mohamed Dhore in the capital says the blast is far bigger than any carried out by Kenyan forces which have recently moved into some of the areas of southern Somalia controlled by al-Shabab.
Eyewitnesses say it was heard many miles away.
Earlier reports said six people had been killed.
The US military, which has a base in neighbouring Djibouti, has previously carried out drone strikes in Somalia.
It has also launched air strikes against alleged al-Qaeda militants in the country.
BBC East Africa correspondent Will Ross says a number of air strikes have been reported in recent months but for now no-one seems prepared to own up to them - possibility because of the potential backlash should civilians be killed.
Somali Prime Minister Abdiweli Mohammed Ali on Thursday called for air strikes against al-Shabab as long as civilians were not harmed.
He was speaking at a major conference in London to discuss ways of ending two decades of conflict in Somalia.
But at the same event, US Secretary of State Hillary Clinton said that air strikes were not a good idea and there was no reason to believe anyone was considering them.
"That air strike is not from our end," Kenyan army spokesman Col Cyrus Oguna told the AFP news agency.
Al-Shabab has cordoned off the site of the attack in the Lower Shabelle region and has not yet commented.
"We heard a very loud explosion, and people are saying the target was a vehicle of al-Shabab," Ahmed Moalim, a resident in a nearby village, told AFP.
Eyewitnesses told the BBC that some of those killed appeared to be of European or Asian origin.
But some local media are reporting that one of the dead was a prominent Kenyan jihadist.
On Thursday, Kenya said it had arrested five Kenyan men and one Tanzanian suspected to have al-Shabab links near the Somalia border.
Col Oguna said it was possible that they had deserted from al-Shabab because of the military pressure inside Somalia.
Our correspondent, Will Ross, says as well as attracting jihadists from Europe and north America in recent years, the Islamist militant group has also recruited young men from across the region.
In many of the Kenyan communities close to the Somali border the poverty is deep and with few opportunities for employment, they have been lured into joining al-Shabab with promises of a better life, he says.