Six US troops in Afghanistan were killed and one wounded in a helicopter crash Tuesday, but it was unclear if Taliban fire caused any of the casualties after the chopper went down.

Military officials said insurgents did not shoot down the UH-60 Blackhawk, but they were investigating whether any of the US troops were killed by gunfire from Taliban militants after the crash.

The Afghan insurgents immediately claimed responsibility for the deaths, using their main Twitter account to report that their fighters had shot down the US chopper in the southern 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.

"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."

Defence officials in Washington told AFP the fatalities were US troops riding in a Blackhawk aircraft.

"I can confirm six Americans were killed," said one official, who spoke on condition of anonymity.

An investigation was underway to determine "the cause of engine failure," the official said.

Although there was no insurgent fire on the aircraft before the Blackhawk crashed, "once it was down, there was enemy fire," said a US military officer, who asked not to be named.

The Taliban Twitter account, under the name Abdulqahar Balkhi, said the helicopter was brought 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 confirmed the incident 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 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.

The last fatal helicopter incident for US forces occurred in April, when an Apache chopper went down in eastern Afghanistan, claiming the lives of two American troops.

Five US troops also died in the southern province of Kandahar in March when their helicopter came down during a heavy rainstorm.

Before Tuesday's crash, there had been 149 NATO fatalities in Afghanistan this year, 119 of them US soldiers, according to the independent icasualties website.

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 the local army and police have made enough progress to provide general security and keep the Taliban at bay.

There are currently 42,700 US troops deployed in Afghanistan as part of the NATO-led force that is due to withdraw by the end of 2014.

A small US-led contingent is due to stay in the country pending the signature of a security agreement between Washington and Kabul.

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