Six soldiers from the US-led NATO mission fighting Taliban insurgents in Afghanistan were killed in a helicopter crash on Tuesday, with NATO officials saying it was not a suspected militant strike.

However the Taliban immediately claimed responsibility for the deaths, using their main Twitter account to report that their fighters had shot down a US helicopter in the province of Zabul.

The incident was the single biggest loss of life for the NATO mission in Afghanistan since seven Georgian soldiers died when a suicide bomber blew up a truck loaded with explosives outside a base in Helmand province in June.

"Six International Security Assistance Force (ISAF) service members died following an aircraft crash in southern Afghanistan today," an ISAF statement said, declining to give the nationalities of the victims.

"The cause of the crash is under investigation, however initial reporting indicates there was no enemy activity in the area at the time of the crash.

"It is ISAF policy to defer casualty identification procedures to the relevant national authorities."

The Taliban Twitter account, under the name Abdulqahar Balkhi, said the helicopter was shot down on Tuesday afternoon while flying low over the district of Shah Joy in Zabul.

"(The) chopper crashed in (a) ball of flame... killing all 8 invaders aboard," the account said.

The Taliban regularly make unsubstantiated claims of attacks on NATO and Afghan forces and also exaggerate casualty numbers in proven strikes.

Provincial officials were unable to immediately confirm the cause of the accident in Zabul, a restive province bordering on Helmand and neighbouring Pakistan.

"I can confirm a helicopter crashed in Shah Joy district this afternoon but we don't have any information about the casualties or the cause of the hard landing," Mohammad Jan Rasolyar, deputy governor of Zabul province, told AFP.

Local officials said that ISAF and Afghan forces rushed to the scene of the crash and were still on patrol around the site when darkness fell.

Aircraft crashes are fairly frequent in Afghanistan, where the 75,000-strong international mission relies heavily on air transport as it battles the insurgency alongside Afghan forces who now take the lead in most military operations.

In April, two helicopter crashes killed a total of six NATO troops, while five US troops died in the southern province of Kandahar in March when their helicopter came down during a heavy rainstorm.

This year there have been 149 NATO fatalities in Afghanistan, 119 of them US soldiers.

The annual total peaked in 2010 when 711 NATO troops died.

NATO combat operations in Afghanistan are due to end next year, and coalition commanders say that the local army and police have made enough progress to provide general security and keep the Taliban at bay.