Parliament will this week be asked to endorse the deployment of British troops in a UN peacekeeping force, as the UK signals its willingness to play a leading role in the reconstruction of Afghanistan.

Geoff Hoon told the BBC that a request from the UN will be taken "very seriously" by the UK government and said parliament would be consulted before troops were committed.

"If the United Nations makes a request, we will look at that very, very seriously," he said. "We are certainly prepared to help with something that is important, as we go on ensuring that Afghanistan has a real future after the tremendous efforts that have been made on the ground in the military campaign. We can't afford to let this opportunity slip."

Following Colin Powell's statement that Britain would play the lead role in a peacekeeping force, the defence secretary said: "I see every advantage in one country providing the main elements of the headquarters of this operation, something that the UK has done very well in the past."

Hoon played down reports that the UK could contribute a contingent as large as 10,000 troops in a force of 50,000 peacekeepers and called on the UN to take an inclusive approach when planning the intervention.

"I don't believe that necessarily any one country will have such a large number," he said. "What is very important is that this is a multinational force and reflects the wide range of support the international community have given to Afghanistan. What is very important is that all those countries who have offered help already are able to participate."

As the search for Osama bin Laden intensifies, Hoon indicated that Britain believes he should be tried in the US. "That is that the country that has the main call on him as far as bringing him to justice is concerned - which clearly is the United States - we should hand him over fairly promptly to face justice in the US," he said.

But he warned that America would have to make "certain undertakings" not to execute the al Qaeda chief before extradition took place.

"We do extradite people to countries with the death penalty, obviously subject to certain undertakings," he said. "I see no reason in principle why that shouldn't happen. I would mean, of course, that certain undertakings would have to be given about any penalty he might face."A UN team has arrived in Kabul to pave the way for an international peacekeeping force over the coming weeks.

A seven-strong technical squad from the UN's department of peacekeeping operations is doing the ground work for a "coalition of the willing" that could number tens of thousands.

Troops are set to be stationed after the interim government, headed by Hamid Karzai, takes office on December 22.

UN spokeswoman Wivina Belmonte said that the size and make-up of any eventual military force had not yet been finalised.Colin Powell said: "There's a lot of discussion about mission, lead, size, connection to the UN (and) connection to the interim administration."