Central Command chief General Kenneth McKenzie said 12 were killed when two suicide bombers believed deployed by the Afghanistan branch of the Islamic State group exploded their bombs by a key gate into the airport and at a nearby hotel used for staging evacuees.

The dead, according to media reports, included 11 US Marines and one medic from the navy.

- Helicopter missions -

The two-decade war has cost 1,909 US military lives in combat. The heaviest losses came on August 6, 2011, when insurgents shot down a Chinook transport on a nighttime mission in Wardak province southwest of Kabul.

Thirty US American service members, including 22 Navy SEALs special operations troops, were killed in the crash, as well as eight Afghans and a US military dog.

Prior to that, the worst single-day toll also involved a helicopter. On June 28, 2005, three Navy SEALs were killed in a firefight after being flown in to the mountains of the eastern Kunar province.

A helicopter loaded with reinforcements that was sent to help one SEAL still alive on the ground and recover the bodies of the three was shot down, killing 16 on board.

- Firefights -

Other major losses include a firefight between scores of Taliban fighters and US troops in Wanat in Nurestan province in July 2008, which saw nine US troops killed.

Fifteen months later, in October 2009, eight Americans died in a similar battle with hundreds of Taliban fighters in Kamdesh, also in Nurestan province.

- Perceived allies -

The war has also seen high tolls from attacks by perceived allies on the Americans.

On April 27, 2011 eight members of the US Air Force and one US civilian were shot dead in Kabul airport by an Afghan pilot.

And on December 30, 2009, a "triple-agent" whom US intelligence thought was on their side killed seven CIA officers and contractors, along with two others, at a CIA facility in eastern Afghanistan known as Camp Chapman.