At least six people have been killed in a helicopter crash in Afghanistan, within hours of a US announcement that two more of its Marines had died in combat.

The Chinook helicopter, which a Nato spokesman described as a civilian aircraft, came down in southern Helmand province where US Marines are carrying out a massive operation against Taleban insurgents. However, it is unclear whether the dead are military personnel or contractors, as an Afghan official who witnessed the incident said that the helicopter was being used by foreign forces.

A spokeswoman for the US Navy said that the crash took place in the Sangin district of Helmand. “We know that there were casualties, but we are still receiving information from the scene and at this time do not have any additional details about them,” she said. The Taleban claims to be responsible for shooting down the Chinook.

Around 4,000 US Marines and hundreds of Nato and Afghan forces are taking part in an offensive in various parts of Helmand against the Taleban, the biggest by foreign troops since they ousted the Islamist group in 2001.

The operation is being staged before next month’s crucial presidential election. Since the launch of the mission in Helmand, the Taleban have stepped up their attacks in other parts of Afghanistan against foreign troops and the Afghan Government, making July the bloodiest month for foreign troops for nearly a year.

The military announced today that two US troops were killed “as a result of a hostile incident in southern Afghanistan" yesterday.

Nato said that the troops were from its International Security Assistance Force. It gave no further details but a spokesman for a separate US-led coalition said that the victims were US nationals.

Prior to that, at least 15 foreign servicemen had been killed since the start of the assault in Helmand, part of the main bastion for Taleban guerrillas and the main drug producing region of Afghanistan, the world’s top heroin supplier.

There have been numerous casualties among US forces in other parts of Afghanistan. Since the start of the month 15 British soldiers have also died. The latest deaths brought the total number of British military fatalities since the start of operations in 2001 to 184 — surpassing the 179 who died in Iraq.

An Italian soldier was also killed today in an an explosives attack that wounded three others in western Afghanistan, according to the defence ministry in Rome. “A patrol was attacked this morning with an explosive device around 50 km [30 miles] from Farah,” a spokesman said. “Three men were wounded while a fourth was killed."

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