Thirty-one US and seven Afghan special forces died when the Taliban shot down their helicopter, officials said Saturday, the biggest single loss yet for foreign troops in a decade-long war. All were killed during an anti-Taliban operation late Friday when a rocket fired by the insurgents struck their Chinook aircraft as it began to take off shortly after a firefight in Wardak province, southwest of the capital Kabul. The death toll was given in a statement from Afghan President Hamid Karzai's office but it was not immediately confirmed by the NATO-led International Security Assistance Force (ISAF). "The president of the Islamic Republic of Afghanistan expresses his sympathy and deep condolences to US President Barack Obama and the families of the victims," it said. The Afghan defence ministry said the local troops who died were members of the country's special forces. Twenty-five of the dead were US Navy SEALs, US television network ABC News reported. The Pentagon declined to comment on the cause or number of deaths. Obama paid tribute to the Americans and Afghans who died and said the incident was a reminder of the "extraordinary sacrifices" made by the men and women of the military and their families US special forces play a key role in the fight against the Taliban by hunting down and killing its fighters in targeted night raids, but Kabul has repeatedly blamed such operations for causing civilian deaths. The Friday night shoot-down was by far the worst incident to hit foreign troops since American and other international forces invaded Afghanistan and ousted the Taliban from power in 2001 in the wake of the September 11 attacks. The previous biggest death toll saw 16 American soldiers killed in 2005 when a Taliban rocket hit their Chinook in the eastern province of Kunar. One man who said he witnessed Friday's crash, Mohammad Saber, told AFP that the helicopter plummeted during a late-night operation in his village. "At around 10:00pm last night (1730 GMT), we heard helicopters flying over us," he said. "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 Chinook was shot down in Sayd Abad district as it was taking off. "A rocket fired by the insurgents hit it and completely destroyed it," he added. The Afghan army commander for the region, General Abdul Razeq, also said the helicopter was "shot down by a rocket fired by the enemy." A spokesman for ISAF said it would issue a statement "at an appropriate moment." NATO Secretary General Anders Fogh Rasmussen pledged that the force was "determined to stay the course," despite the latest deaths. 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. A Western military source speaking on condition of anonymity confirmed the helicopter type. Chinooks are widely used by coalition forces in Afghanistan for transporting large numbers of troops and supplies around the war zone. There are currently about 140,000 foreign soldiers in Afghanistan, around 100,000 of them from the US, but all combat forces are due to leave by the end of 2014. Some troop withdrawals have already begun as part of a transition that has already seen Afghan security forces take control of key regions this summer. A total of 342 foreign troops -- 279 from the United States -- have now been killed in Afghanistan this year, according to an AFP tally based on the independent website iCasualties.org. Foreign troop commanders say the east of the country, close to Pakistan where insurgents have hideouts, will likely increasingly overtake the south as the focus of the war in coming months.