The US military says the situation at a Kenyan airfield used by US forces is ``fluid`` as fighting reportedly continues.
A statement by the US Africa Command says the Manda Bay airfield ``is still in the process of being fully secured.`` It does not say whether any US or Kenyan forces were killed.
The Somalia-based al-Shabab extremist group has claimed responsibility and asserts that intense combat continues. The US statement calls the al-Shabab claims exaggerated and says US and Kenyan forces repelled the attack.
This is 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 Somali border 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 signaled 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 a.m. 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.
Kenya's military, however, said that ``the airstrip is safe.`` It said that ``arising from the unsuccessful breach a fire broke out affecting some of the fuel tanks located at the airstrip.`` The Kenya Civil Aviation Authority said the airstrip was closed for all operations.
Al-Shabab, which is linked to al-Qaida, has launched a number of attacks inside Kenya, including against civilian targets such as buses, schools and shopping malls.
The group has been the target of a growing number of US airstrikes inside Somalia during President Donald Trump's administration.
The latest attack comes just over a week after an al-Shabab truck bomb in Somalia's capital killed at least 79 people and US airstrikes killed seven al-Shabab fighters in response.
Last year al-Shabab attacked a US military base inside Somalia that is used to launch drone strikes. The extremist group also has carried out multiple attacks against Kenyan troops in the past in retaliation for Kenya sending troops to Somalia to fight it.
This attack marks a significant escalation of al-Shabab's campaign of attacks inside Kenya, said analyst Andrew Franklin, a former US Marine and longtime Kenya resident.
``Launching a deliberate assault of this type against a well-defended permanent base occupied by (Kenya Defence Forces), contractors and US military personnel required a great deal of planning, rehearsals, logistics and operational capability,'' he said. Previous attacks against security forces have mainly been ambushes on Kenyan army or police patrols.
Analyst Rashid Abdi in Twitter posts discussing the attack said it had nothing to do with the tensions in the Middle East but added that Kenyan security services have long been worried that Iran was trying to cultivate ties with al-Shabab.
``Avowedly Wahhabist Al-Shabaab not natural ally of Shia Iran, hostile, even. But if Kenyan claims true, AS attack may have been well-timed to signal to Iran it is open for tactical alliances,`` he wrote, adding that ``an AS that forges relations with Iran is nightmare scenario.``
When asked whether the US military was looking into any Iranian link to the attack, spokesman Karns said only that ``al-Shabab, affiliated with al-Qaida, has their own agenda and have made clear their desire to attack US interests.``