Three Americans - one U.S. military servicemember and two contractors - were killed by Somalia's al Shabaab militant group during an attack on Sunday on a military base in Kenya used by both U.S. and Kenyan forces, the U.S. military said.
The military's Africa Command confirmed the deaths and said two other Americans who work for the U.S. Department of Defense were also wounded in the attack on the Manda Bay Airfield in Lamu county, close to the Somali border.
"The wounded Americans are currently in stable condition and being evacuated," Africa Command (AFRICOM) said in a statement.
The attack presents another crisis for Washington just as the Pentagon grapples with a rapidly escalating standoff with Iran following a Friday U.S. drone strike in Baghdad that killed top Iranian military commander Qassem Soleimani.
Tehran and Washington have traded threats and counter-threats following the strike, stoking fears of open conflict.
The assault by al Shabaab, which has been fighting for more than a decade to overthrow the Somali government and impose strict Islamic law, began before dawn and lasted around four hours, witnesses and military sources told Reuters.
A Kenyan police report seen by Reuters said the Islamist militants destroyed two planes, two U.S. helicopters and multiple American military vehicles during their assault.
The Kenyan military said five militants had been killed in the attack. There were no immediate reports of Kenyan casualties.
In a statement earlier on Sunday, al Shabaab claimed it had destroyed seven aircraft and three military vehicles, without providing other details. It also published pictures of masked gunmen standing next to an aircraft in flames.
AFRICOM said fewer than 150 U.S. personnel had been at the base, where they provided training and counterterrorism support to East African forces.
"Alongside our African and international partners, we will pursue those responsible for this attack," said U.S. Army General Stephen Townsend, who leads Africa Command.
Kenyan military spokesman Colonel Paul Njuguna said the base had been secured.
"This morning at around 5:30 am an attempt was made to breach security at Manda Air Strip. The attempted breach was successfully repulsed," he said in a statement.
"Arising from the unsuccessful breach a fire broke out affecting some of the fuel tanks located at the airstrip. The fire has been put under control."
In the operation to repulse the attack, at least five militants were killed and weapons including four AK47 rifles were seized, Njuguna said.
There was no indication the militants had managed to enter the base. The airfield is separate to another on Manda Island used by commercial flights to Lamu.
Kenya sent troops into Somalia in 2011 after a spate of cross-border attacks and kidnappings. They were later absorbed into an African Union peacekeeping force, now 21,000-strong, which supports the shaky, Western-backed Somali government.
EXPLOSION IN THE DARK
Independent investigator Benjamin Strick, who analyses satellite imagery for open-source investigation websites such as Bellingcat, said the photos of gunmen next to a burning plane published by al Shabaab matched satellite images of buildings and a distinctive aircraft apron adjacent to the base but outside its perimeter.
Residents on nearby Lamu Island, a haven for wealthy tourists and visiting European royalty, said a loud explosion jolted them awake before 4 a.m.
Abdalla Barghash said he later saw a large dark plume of smoke rising from the Manda Bay mainland, where the airstrip and base are located.
Lamu county, which is far more impoverished than the island, is frequently targeted by al Shabaab with roadside bombs and ambushes on travellers or attacks on isolated villages.
The insurgents killed three passengers when they attacked a bus in the county on Thursday.