The US and Afghan forces came under attack in eastern Afghanistan on Saturday, as they launched investigations into what the New York Times described as a deadly shootout between Afghan and American soldiers during a joint exercise.

The Times, quoting two Afghan officials, reported that five or six American soldiers and six Afghan soldiers were killed.

The newspaper also quoted a US military official saying there were at least six American casualties and confirmed that there were fatalities without saying how many.

Colonel Sonny Leggett, a spokesman for U.S. forces in Afghanistan, said a combined U.S. and Afghan force conducting an operation in Nangarhar province was engaged by direct fire.

Officials in Afghanistan did not comment on the number of casualties.

“We are assessing the situation and will provide further updates as they become available,” he said.

According to the US officials, it is not clear whether militants in Afghan uniforms attacked the US forces in Nangarhar province or whether there was an argument between the allies.

Mubariz Khadem, a senior security official in Nangarhar, said the clashes took place between US and Afghan forces and casualties were feared.

Insider attacks, often known as “green-on-blue” attacks, have been a regular feature of the conflict in Afghanistan, although their frequency has diminished in recent years.

Sohrab Qaderi, a provincial council member in Nangarhar, said clashes happened between the Afghan army and foreign forces in Shirzad district on Saturday afternoon.

Some 13,000 US troops remain in Afghanistan after the US intervention to oust the Taliban following the 11 September 2001 attacks.

The Taliban now control more territory than at any point since they were ousted in 2001, the BBC reported.

Peace talks between the US and the militants have stalled in recent months.

(With inputs from agency)