Britain is not being drawn into a new Vietnam by increasing troop numbers in Afghanistan, Number 10 has insisted.

Speaking after defence secretary, Geoff Hoon, announced that 1,700 Royal Marines are heading to Afghanistan, Downing Street rejected comparisons with the Vietnam conflict.

The comments came as the former Labour minister Peter Kilfoyle warned the government that it was being drawn into a long and bloody conflict.

Tony Blair's official spokesman repeated that Britain had joined the war on terrorism following the September 11 attacks and would not walk away when things got tough.

"It's important to go back six months ago. I don't think the British public's memories are short. We had the most murderous terrorist attack in history. That is why we are involved here and that is why we must never forget it," said the official spokesman.

But Number 10 admitted that military intelligence had still to discover the location of Osama bin Laden or his accomplice Mullah Omar.

"We don't know and we aren't even going to pretend that we know," said the spokesman.

There was also the stark warning that there would be British casualties.

"I don't think anyone is under any illusion. We are not talking peacekeeping here, we are talking warfare. It is dangerous terrain, testing conditions, and a very murderous enemy." said the spokesman. "We have to acknowledge the real risk to our forces."

"This is moving to a new phase. What our involvement means is that the military initiative inside Afghanistan will be more flexible. It will be more responsive to the situation there. It will be able to exert more pressure, which as a result should hopefully mean more success more quickly."

Claims that British marines were being called in to shore up inadequate US troops were angrily dismissed by the government.

Number 10 maintained that the operation had been a success and stressed that the marines were being brought in to maintain momentum.

"What you are seeing is the green light given to a commitment which was always there in principle from the outset," he said.

Criticism of American troops was "a slur on their professionalism".

"We are far, far further forward than anyone could have predicted at the start of this campaign."