At least four people were killed and 14 others injured in an explosion inside the largest US military base in Afghanistan, Nato said, with the Taliban claiming responsibility.

The explosion struck at dawn inside the heavily fortified Bagram Airfield north of Kabul, as the Taliban step up attacks on Western targets before the onset of winter, when fighting usually ebbs.

The nationalities of those killed were not immediately known after the blast, which highlights a worsening security situation nearly two years after Nato formally ended its combat operations in Afghanistan.