NAIROBI/WASHINGTON－Three US citizens－one military serviceman 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 US and Kenyan forces, the US military said.

The US military's Africa Command confirmed the deaths and said two other US citizens who work for the US 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, or 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 US drone strike in Baghdad that killed senior Iranian military commander Qassem Soleimani.

Teheran and Washington have traded threats and counterthreats following the strike, stoking fears of open conflict.

The assault by the Islamist insurgent al-Shabaab group, which has been fighting for more than a decade to overthrow the Somali government, began before dawn and lasted around four hours, witnesses and military sources told Reuters.

A Kenyan police report seen by Reuters said al-Shabaab militants destroyed two planes, two US helicopters and multiple 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 US 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 US 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 AK 47 rifles were seized, he 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.