NAIROBI, Kenya — The U.S. military said the security situation was “fluid” at a Kenyan airfield used by U.S. forces after a pre-dawn attack Sunday by the al-Shabab extremist group. The attack destroyed U.S. aircraft and vehicles, Kenyan authorities said, and at least four attackers were killed.

It was not yet clear whether any U.S. or Kenyan forces were killed. A U.S. Africa Command statement said "an accountability of personnel assessment is underway." The midday statement said the Manda Bay airfield was "still in the process of being fully secured."

The al-Qaida-linked al-Shabab, based in neighboring Somalia, claimed responsibility and asserted that "intense" combat with U.S. forces continued. The U.S. statement called the al-Shabab claims exaggerated and said U.S. and Kenyan forces repelled the attack.

It is the first known al-Shabab attack against U.S. 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. Residents said a car bomb had exploded early in the morning. Lamu county commissioner Irungu Macharia told The Associated Press that five suspects were arrested and were being interrogated.

An internal Kenyan police report seen by the AP said two fixed-wing aircraft, a U.S. Cessna and a Kenyan one, were destroyed along with two U.S. helicopters and multiple U.S. 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.

The U.S. military said only that "initial reports reflect damage to infrastructure and equipment." The Kenya Civil Aviation Authority said the airstrip was closed for all operations.

Al-Shabab's statement included 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 U.S. forces continued.

The military's Camp Simba in Lamu county, established more than a decade ago, has under 100 U.S. personnel, according to Pentagon figures. U.S. forces at the Manda Bay airfield train and give counterterror support to East African partners, according to the military. A U.S. flag-raising at the camp in August signaled its change "from tactical to enduring operations," the Air Force said at the time.

According to another internal Kenyan police 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 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 U.S. 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 U.S. airstrikes killed seven al-Shabab fighters in response.

Last year al-Shabab attacked a U.S. military base inside Somalia, Baledogle, that is used to launch drone strikes but reportedly failed to make their way inside. 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 U.S. 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 U.S. 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.

The early Sunday attack comes days after a U.S. airstrike killed Iran's top military commander and Iran vowed retaliation, but al-Shabab is a Sunni Muslim group and there is no sign of links to Shiite Iran or proxies.

"No, this attack was no way related to that incident" in the Middle East, an al-Shabab spokesman told the AP. He spoke on condition of anonymity for security reasons.

One analyst, Rashid Abdi, in Twitter posts discussing the attack agreed 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."

But a former member of the U.N. committee monitoring sanctions on Somalia, Jay Bahadur, said in a tweet that "the attack is far more related to AS wanting a do over on their spectacular failure at Baledogle four months ago."

When asked whether the U.S. military was looking into any Iranian link to the attack, U.S. Africa Command spokesman Col. Christopher Karns said only that "al-Shabab, affiliated with al-Qaida, has their own agenda and have made clear their desire to attack U.S. interests."

The al-Shabab claim of responsibility said Sunday’s attack was part of its “Jerusalem will never be Judaized” campaign, a rarely made reference that also was used after al-Shabab’s deadly attack on a luxury mall complex in Kenya’s capital, Nairobi, in January 2019.