Five US troops and an Afghan officer have been killed in what is suspected to be one of the worst incidents of "friendly fire" in the 13-year-old war in Afghanistan.

The US department of defence said on Tuesday that its soldiers were killed during an operation in the southern Zabul province the previous day.

We do have reason to suspect ... friendly fire from the air. John Kirby, Pentagon spokesman.

"We do have reason to suspect friendly fire was cause here, specifically friendly fire from the air," said John Kirby, a spokesman. A source later told the AP news agency that all were members of the special forces.

Ghulam Sakhi Rooghlawanay, a provincial police chief, said the deaths were caused by an air strike during a battle with Taliban fighters.

"The joint forces came under attack by insurgents, and then foreign forces called for air support," he told the AP news agency. "Unfortunately five NATO soldiers and one Afghan army officer were killed mistakenly by NATO air strike."

The news agency said there was no way to independently confirm Rooghlawanay's comments.

NATO commands the International Security Assistance Force in Afghanistan.

If proved correct, the deaths on Monday would be among the worst incidents of fratricide in the war in Afghanistan.

In April 2002, four Canadian soldiers were killed by a bomb dropped by a US F-16 during night exercises in Kandahar.

Three British soldiers were killed in August 2007 when a US F-15 dropped a bomb on their position in Helmand.

On November 26, 24 Pakistani soldiers were killed when ISAF soldiers fired on their border position, believing them to be Taliban fighters.

All US combat troops are scheduled to be withdrawn from the country by the end of this year.