At least four people have been killed and 14 more wounded in a bomb attack at a US airfield in Afghanistan, the head of international military operations in the country has said.

The explosion in the early hours of Saturday was caused by an “explosive device”, US Army General John Nicholson confirmed, although he did not give further details.

“Force protection and medical teams are responding to the situation,“ Nato said in a statement, and an investigation has been opened.

Bagram, which doubles as a large detention centre, is frequently targeted by Taliban militants with missiles. A spokesperson for the group claimed the explosion was caused by a suicide bomber who managed to get inside the base after planning the incident for the last four months.

Abdul Wahid Sediqqi, spokesman for the governor of Parwan province, where the air field is based, told the Associated Press that local employees usually line up at the base’s gates around dawn, adding that an attacker could have been among the men who entered on Saturday morning.

There was no immediate word on the nationalities of those killed and wounded. It is Nato policy not to release such details until families have been informed.

The Taliban have waged an insurgency war against successive US-backed governments since they were removed from power in 2001.

The group currently controls more territory in Afghanistan than at any point in the last 15 years, the UN says.

Alleged abuse of prisoners at Bagram was documented in the film Taxi to the Dark Side, which told the true story of the torture and death of a taxi driver at the hands of US troops.