By Mark Davies BBC News Online political reporter

As words of warning go, it could hardly be stronger.

Asked about the government's announcement that more UK troops are joining the US-led operation in Afghanistan, a former defence minister drew comparisons with the Vietnam war.

The fear that British forces will be drawn into a long-lasting, bloody conflict is one of a number of concerns being expressed at Westminster following Defence Secretary Geoff Hoon's Commons statement on Monday.

It is a very murky, messy picture that we are putting our troops into

Peter Kilfoyle MP

Former defence minister Peter Kilfoyle is not the only person to have used the "V-word" - Vietnam - in relation to Mr Hoon's announcement.

And Liberal Democrat foreign affairs spokesman Menzies Campbell spoke for many on Tuesday when he said the government needs to "fill in the blanks" over the announcement.

Mr Kilfoyle said the mood on the backbenches was one of "extreme concern".

It is partly the nature of the announcement, which came as a surprise to MPs, which has stirred feelings at Westminster.

Against a background of growing concern about the possibility of British involvement in US-led action against Iraq, the timing unsettled backbenchers.

Ministers have, however, been careful over recent months to stress that when it comes to the war in Afghanistan, the UK is "in for the long haul".

Hoon: Announcement "consistent" with aims and objectives

Last autumn, the UK's chief of defence staff, Admiral Sir Michael Boyce warned that military action in Afghanistan could continue well into next year.

It is what he went on to say which is providing cause for concern among some MPs.

Any deployment of ground troops, said Sir Michael, would probably be only for a short period, with very clearly defined objectives.

Cause for concern

"There needs to be a very clear reason for going in. It is not a country, as history has told us, for us to linger in, other than to go in for a very specific task and then probably withdraw again," he said then.

That goes to the heart of what is worrying some MPs.

They say the reasons for more UK troops joining the operation now are not clear enough, that the objectives of the operation are not specific enough and that an exit strategy is not apparent.

The fear is that the UK's commitment to the operation appears open-ended.

Defence sources have spoken about any operation to make Afghanistan safe from Taleban or al-Qaeda forces taking years.

Questions are also being raised about the command structure of the operation, which is led by the US.

'Messy picture'

And with the Iraq issue rumbling in the background, some MPs are concerned that the government is meeting US requests too readily.

Sir Michael had warned of a long campaign

Mr Kilfoyle drew comparisons with Vietnam, saying the then Prime Minister Harold Wilson has resisted US pressure to send troops to aid US forces.

"It is a very murky, messy picture that we are putting our troops into," he said.

Mr Hoon, however, and with some justification, says his announcement is "entirely consistent" with the objectives set out since 11 September.

UK troops, he said, will flush out the remnants of al-Qaeda forces and will leave Afghanistan when that mission is completed.

Ministers have warned repeatedly - though not as regularly recently as it appeared that the war in Afghanistan was drawing to a close - that Britain would be involved for some time.

'Time and patience'

Last October, Downing Street warned that the war on terrorism will be a long and difficult task, while Foreign Secretary Jack Straw said the Afghanistan campaign could last "indefinitely".

Mr Blair's spokesman called for "time and patience".

He added: "We do have a clear strategy". He said Mr Blair had always seen the process as a "long haul and a hard grind".

And Sir Michael warned that coalition forces probably face a long struggle.

"It could be a very short haul, but we must expect to go through the winter and into next summer at the very least," he said then.

For the UK troops preparing to head for the battle zones in Afghanistan, that prediction is set to become a reality.