A suicide bomber dressed as a labourer blew himself up on Saturday at the largest American military base in Afghanistan, killing four Americans, said a report, citing Defence Secretary Ash Carter.

Two members of the armed services and two contractors died in the attack inside Bagram airbase, according to BBC.

A further 16 US service members and one Polish soldier were injured, he added.

The Taliban said one of their fighters had carried out the attack, a major security breach in one of the best protected places in Afghanistan.

Carter said he was "deeply saddened" by the US casualties and promised an investigation.

Bagram has been targeted by militants in the past but this is the first time a bomb has exploded inside the base.
Just north of the capital Kabul, it has been used as the main military base and airfield by the US-led forces and Nato over the past 14 years.

A local government spokesman said the attacker had entered the base early in the morning and was among Afghan labourers reporting for duty when he detonated his vest.