The strike brings the number of Nato troops who have died this week to 21.

American helicopter

Al Jazeera’s James Bays, in the capital Kabul, said there had been very heavy fighting in Helmand in recent weeks.

"Sangin is the area of British operations, although I discovered from my sources that the helicopter that was brought down was an American helicopter," he said.

IN DEPTH

 

  Blog: The Afghan peace plan
  Video: Taliban fighters reject peace offer
  Riz Khan: Is Afghanistan a failing state?
  Inside Story: The Taliban's counter-strategy

"Always at this time of the year there's more Taliban activity. It's what they call the 'fighting season'. It happens every year.

"But already this year, 253 Nato soldiers have been killed, and we're only just at the start of the fighting season."

Bays said 520 Nato troops were killed in Afghanistan last year.

Another Nato soldier was reported to have been killed in a separate attack on Wednesday.

The attacks come a day after 10 troops from the US-led forces in Afghanistan were killed in separate fighting.

US and British forces are stationed in southern Afghanistan, the Taliban's primary area of activity, and are planning a major operation in Kandahar in the south.

In December 2009, Barack Obama, the US president, said that 30,000 more US troops would be deployed to the country, primarily in the south, to try and end the war there.

The US military entered Afghanistan in 2001 to remove the Taliban from power, whom they accused of harbouring al-Qaeda operatives, following the September 11, 2001 attacks in the US.