Geoff Hoon, UK defence secretary, defended his decision to send 1,700 troops into Afghanistan to help battle the remnants of al-Qaeda and Taliban forces.

Mr Hoon described in a BBC radio interview the country's commitment in Afghanistan as "open-ended" and said the exact duration of the operation presented on Monday in the House of Commons would depend on the success achieved in rooting out Taliban and al-Qaeda fighters. "We have to deal with them, otherwise they will continue to pose a threat not only to the people of Afghanistan but also to the people of the UK," he warned.

However, Mr Hoon admitted he did not know the exact whereabouts of Osama bin Laden or how many of his supporters were still hiding out in the mountains of Afghanistan.

Peter Kilfoyle, former UK defence minister, expressed concern about the prospect of British troops being enmeshed in Afghanistan. He said, "It is a very murky, messy picture we are putting our troops into."

The force - based around 45 Royal Marine Commando - will be under US command and will form a brigade alongside US and Canadian battlegroups.

It is Britain's biggest deployment for combat operations since the Gulf war and underscores that while the US has characterised the fighting in Afghanistan as "winding down", the war is still going strong.

The UK troops will join 5,300 US troops already on the ground in Afghanistan. There are no plans to reduce that number.

General John Rosa, of the US joints chiefs of staff, said on Monday he would not characterise the new deployment as marking a new escalation of the war. "I don't think you could characterise it as a major push," he said.

But a British embassy official said the recent fighting in Afghanistan had clearly shown the need for more regular troops to be involved in taking out any remaining opposition in the country. "The nature of the campaign in Afghanistan has obviously gone in a more boots-on-the-ground direction," he said.

Mr Hoon admitted in the House of Commons: "These troops are being deployed to take part in war fighting operations. We will be asking them to risk their lives." But he said there were limits on the amount of time British soldiers could spend fighting in Afghanistan and did not rule out the possibility of replacing the detachment if the troops had not succeeded in their mission in three months.

The announcement came as the US on Monday declared an end to Operation Anaconda, the two-week push against up to 1,000 al-Qaeda and Taliban fighters in south-east Afghanistan.The operation was the biggest of the war.

Pentagon officials said on Monday that 16 al-Qaeda and Taliban fighters were killed after US forces spotted them in vehicles 45 miles south-west of Gardez. There were no US casualties.

Mr Hoon refused to answer questions about whether the deployment made it less likely the UK would have the resources to take part in any action in Iraq.

"The force will go initially to Bagram, with the first members on the ground within days and ready to commence operations by mid-April." Three companies of 45 Commando are already on board HMS Ocean in the Indian Ocean, with the remaining two held at high readiness in Arbroath, Scotland.