Somali Islamists will avenge the raid in which a top al-Qaeda suspect was reportedly killed in Somalia, an al-Shabab commander has told the BBC.
Kenyan-born Saleh Ali Saleh Nabhan is believed to have been killed in a US military helicopter raid on Monday.
US agents have been hunting Nabhan for years over attacks on a hotel and an Israeli airliner in Kenya in 2002.
It is believed he fled to Somalia after the Mombasa attacks and was working with the al-Shabab group.
The al-Shabab commander, who spoke on condition of anonymity as he was not authorised to speak publicly, said the insurgents would retaliate against US interests.
"They will taste the bitterness of our response," he told the BBC's Mohammed Olad Hassan in the capital, Mogadishu.
Somali sources told the BBC that six helicopters were involved in the attack on Monday afternoon on two vehicles in the southern coastal town of Barawe, which is controlled by al-Shabab.
A US official was quoted by Reuters news agency as saying special forces had flown by helicopter from a US Navy ship and fired on a vehicle that they believed was carrying Nabhan - whose body was then taken into US custody.
His wife has told the BBC from the Kenyan port town of Mombasa that she has begun the customary 40-day period of mourning.
Our reporter says the raid has raised concern among Somalis.
They fear such attacks by foreign forces may help to fuel extremism, instead of combating it, our correspondent says.
The US last launched a major strike in Somalia in May 2008, killing al-Shabab's military leader and at least 10 others.
The raid led to protests by villagers and critics say it had little effect on al-Shabab's capabilities.
Analysts say Nabhan is one of the most senior leaders of al-Qaeda's East Africa cell.
Frank Gardner, BBC News
This latest US raid into Somalia, carried out by Joint Special Operations Command, would have had several aims.
Firstly, it would be about "settling scores" - killing or capturing a man the FBI believes was instrumental in al-Qaeda's attacks in East Africa.
A second aim would be to show al-Qaeda's senior operatives that there is no safe hiding place, even in a country whose militants effectively drove out US forces 15 years ago.
Thirdly, the raid's planners would be hoping to throw both al-Qaeda and al-Shabab off-balance, disrupting their plans. Although Nabhan's loss will be felt, reports of his death at US hands are almost certain to trigger revenge attacks in the region.
He is suspected of bombing an Israeli-owned hotel in the Kenyan port city of Mombasa, and trying to shoot down an Israeli airliner in 2002.
The authorities in Kenya also regard him as a suspect in two attacks on US embassies in the region in 1998.
US-based Somalia expert Andre le Sage told the BBC's Network Africa programme that Nabhan's death, if confirmed, would severely hamper the network's ability to operate in the region.
But he said new leaders would probably emerge to take Nabhan's place.
BBC defence correspondent Nick Childs says the raid seems to be something of a departure from recent US tactics in Somalia, which have tended to use long-range missile strikes and aircraft to try to get at militant suspects.
Earlier reports had quoted witnesses as saying the troops wore uniforms with French insignia and had flown from a ship bearing a French flag.
But the French military strongly denied involvement.
US ACTION IN SOMALIA
1992-1994 Sends troops in under UN humanitarian force, gets drawn into clan conflict
3-4 October 1993 Fights brutal battle in Mogadishu - 18 US troops and hundreds of Somalis killed, US helicopters shot down
25 March 1994 Pulls all troops out of Mogadishu
2006-2009 Reportedly supports invading Ethiopian troops fighting Islamists
January 2007 Carries out air strikes targeting al-Qaeda suspects
1 May 2008 Air strike kills al-Shabab military commander Aden Hashi Ayro
June 2009 US confirms it has sent weapons to Somali government
During 2007 and 2008 the US carried out air strikes against Somali Islamist groups it accused of links to al-Qaeda.
The US and France both have troops stationed in neighbouring Djibouti.
Monday's assault comes several weeks after a French security adviser held by militants in Mogadishu managed to get free. A colleague seized at the same time remains in captivity.
Somalia has not had a functioning central government since 1991.
Rival Islamist factions are battling forces loyal to the weak UN-backed government, which controls only small parts of the capital Mogadishu.
Al-Shabab are said to have links to al-Qaeda, and to have been reinforced with foreign fighters.