The Taliban claimed responsibility for the incident, saying it had shot the helicopter down. - AP (File Photo)

PULI ALAM, Afghanistan: Thirty-one US Special Forces and seven local troops were killed when the Taliban shot down their helicopter, officials said Saturday, and in what was the deadliest single incident for foreign soldiers since the war began in 2001.

The Chinook helicopter was downed late Friday during an anti-Taliban operation in an insurgent-infested district of the eastern province of Wardak, just southwest of the Afghan capital Kabul.

Although Western and Afghan officials said they were still trying to assess exactly what happened, an eyewitness claimed it was struck during a firefight.

The Taliban claimed responsibility for the incident, saying it had shot the helicopter down.

The death toll was given in a statement issued by Afghan President Hamid Karzai's office and was not immediately confirmed by the Nato-led International Security Assistance Force (ISAF).

“The president of the Islamic Republic of Afghanistan Hamid Karzai expressed condolences over a Nato helicopter crash and the deaths of 31 members of US Special Forces,” the statement said.

It added that seven Afghans were also killed in the crash, who the country's defence ministry said were also members of the Special Forces.

“The president of the Islamic Republic of Afghanistan expresses his sympathy and deep condolences to US President Barack Obama and the family of the victims,” the statement added.

The strike was by far the worst to hit foreign troops in the near decade-long war. The previous worst saw 16 American soldiers killed on June 28, 2005 when a Taliban rocket hit their Chinook in the eastern province of Kunar.

One man who said he saw Friday's shootdown, Mohammad Saber, told AFP that the helicopter plummeted during an operation in his village.

“At around 10:00pm last night (1730 GMT), we heard helicopters flying over us,” he said.

“We were at home. We saw one of the helicopters land on the roof of a house of a Taliban commander, then shooting started.

“The helicopter later took off but soon after taking off it went down and crashed. There were other helicopters flying as well.” Wardak provincial spokesman Shahidullah Shahid said the crash came during a late-night operation against Taliban insurgents who have been waging war on pro-government forces since 2001.

“The joint forces (foreign and Afghan) conducted an operation against the Taliban in Sayd Abad district last night,” he said.

“Eight insurgents were killed. A coalition helicopter that was firing on insurgents at the time crashed.

“We're not sure if this happened due to insurgent fire or not but there were some casualties of foreign troops due to the crash.” A spokesman for ISAF, the foreign military force in Afghanistan, said they would issue a statement “at an appropriate moment.”

Taliban spokesman Zabiullah Mujahid said the insurgent group was responsible for shooting down the helicopter, which he said was an American Chinook, and acknowledged that eight insurgents had been killed.

The Taliban are known to exaggerate claims in relation to attacks.

Chinooks are used for transporting larger numbers of troops and supplies around in the warzone of Afghanistan.

A Western military source speaking on condition of anonymity confirmed that the helicopter was a Chinook.

There are currently around 140,000 foreign troops in Afghanistan, around 100,000 of them from the United States, fighting as part of the international force that has been in the country for almost 10 years.

Some troop withdrawals have already begun as part of a process which is due to see all foreign combat forces leave the country by the end of 2014.

However, the Taliban are still waging a bloody insurgency in the country.

In recent weeks, a string of high-profile figures close to Karzai have been assassinated.

And in June, the United Nations said that the number of security incidents between March and June was up over 50 per cent on the same period last year.