Casualties as explosion hits largest US base in Afghanistan

No militant group has so far claimed responsibility for the blast, which occurs as the Taliban step up nationwide attacks before the onset of winter, when fighting usually ebbs.

"We can confirm that there was an explosion at Bagram Airfield this morning just after 5:30 am (0100 GMT). There have been casualties," the military coalition said in a brief statement.

"Our Force Protection and medical teams are responding to the situation," it added, without offering any other details.

The explosion underscores a worsening security situation nearly two years after NATO formally ended its combat operations in Afghanistan.

Bagram Airfield, close to Kabul, has frequently come under attack by Taliban insurgents.

Last December, a motorcycle-riding Taliban suicide bomber killed six US soldiers near Afghanistan's largest US military base.

It was one of the deadliest attacks on foreign troops in the country in 2015.