Somalia's al-Shabab extremist group attacked a military base used by US and Kenyan troops in coastal Kenya early on Sunday, destroying US aircraft and vehicles, Kenyan authorities said.
Kenya's military said the pre-dawn assault was repulsed and at least four attackers were killed.
It was the first known al-Shabab attack against US forces inside Kenya, a key base for fighting one of the world's most resilient extremist organizations.
A plume of black smoke rose above the base near the border with Somalia, where al-Shabab is based.
Residents said a car bomb had exploded.
Lamu county commissioner Irungu Macharia told The Associated Press that five suspects were arrested and were being interrogated.
The US Africa Command confirmed the attack on Camp Simba in Lamu county.
Spokesman Col. Christopher Karns called al-Shabab's claims, including that its attack inflicted severe casualties, “grossly exaggerated."
There was no report of US or Kenyan deaths.
The camp, established more than a decade ago, has under 100 US personnel, according to Pentagon figures.
A US flag-raising there in August signalled its change “from tactical to enduring operations," the Air Force said at the time.
An internal Kenyan police report seen by the AP said two fixed-wing aircraft, a US Cessna and a Kenyan one, were destroyed along with two US helicopters and multiple US vehicles at the Manda Bay military airstrip.
The report said explosions were heard at around 5:30 am from the direction of the airstrip.
The scene, now secured, indicated that al-Shabab likely entered “to conduct targeted attacks," the report said.
According to another internal report seen by the AP, dated Friday, a villager that day said he had spotted 11 suspected al-Shabab members entering Lamu's Boni forest, which the extremists have used as a hideout.
The report said Kenyan authorities did not find them.
Al-Shabab's claim of responsibility said Sunday's attack destroyed US equipment including aircraft and vehicles, and it posted photos of blazing aircraft it asserted were from the attack.
A second al-Shabab claim issued hours later asserted that "ïntense close-quarters combat" against US forces continued.