Al-Shabaab extremists have overrun a key military base in Kenya, killing three American Department of Defense personnel and destroying several US aircraft and vehicles before they were repelled.

The attack on the Manda Bay airfield early on Sunday was the al-Qaida-linked group’s first attack against US forces in the East African country, and the military called the security situation “fluid” several hours after the assault.

Five attackers were killed, Kenyan military spokesman Paul Njuguna said.

Al-Shabaab, based in neighboring Somalia, claimed responsibility for the assault.

One US serviceman and two contractors with the US Department of Defense were killed in the fighting, according to a statement from the US Africa Command, or Africom.

Al-Shabaab claimed that there were 17 US casualties, nine Kenyan soldiers killed and seven aircraft destroyed.

Kenya is a key base for fighting al-Shabaab, one of the world’s most resilient extremist organizations. A large plume of black smoke rose above the airfield Sunday and 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.

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 5.30am from the direction of the airstrip. The scene, now secured, indicated that al-Shabaab likely entered “to conduct targeted attacks”, the report said.

The military’s Camp Simba has under 100 US personnel, according to Pentagon figures. US forces at the adjoining Manda Bay airfield train and give counterterror support to East African partners.

Al-Shabaab 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 Donald Trump’s administration.

The latest attack comes just over a week after an al-Shabaab truck bomb in Somalia’s capital killed at least 79 people and US airstrikes killed seven al-Shabaab fighters in response.

Last year, al-Shabaab attacked a US 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-Shabaab’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.

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