Three US service members and a contractor have been killed by an improvised explosive device close to a major base near Afghanistan's capital, Kabul, the US military has said.
The blast took place on Monday close to Bagram Airfield, the largest US military facility in Afghanistan.
In a statement, the military said three other US troops were wounded in the explosion and were receiving treatment.
The Taliban has claimed responsibility for the attack and described it as a suicide car bombing.
The group said an attacker detonated his explosives-laden vehicle near the base in Parwan province.
It is one of the deadliest recent attacks against US personnel. In November, a roadside bomb blast killed three US service members near the central Afghan city of Ghazni.
Last month two US service members were killed in Afghanistan while conducting an operation. The war has taken a much larger toll on Afghan security forces and civilians.
President Ashraf Ghani, speaking at the World Economic Forum in January, said that about 45,000 Afghan security forces have been killed since he took office in September 2014, which works out to an average of 849 per month.
There are about 14,000 US forces in Afghanistan.
