LONDON - Britain's Foreign Secretary David Miliband and its top commander in Afghanistan on Sunday tried to revive support for the fight against the Taliban as thoughts on Remembrance Sunday turned to those killed.

British public support for the war in Afghanistan is falling, while more than 40 percent do not understand why the troops are fighting there, a ComRes poll showed.

Some 64 percent agreed that "the war in Afghanistan is unwinnable", up six percent from July, while 27 percent disagreed, down four percent.

Similar numbers said British forces should be withdrawn as quickly as possible, with 63 percent agreeing and 31 percent disagreeing.

Some 54 percent felt they had "a good understanding of the purpose of Britain's mission in Afghanistan", with 42 percent disagreeing.

Adding to the pressure on British Prime Minister Gordon Brown as the nation pauses to remember the fallen, The Independent on Sunday newspaper called for a withdrawal of British forces.

"The threat is hard to see and more complex to understand" than previous wars, Miliband wrote in The Mail on Sunday newspaper.

"This war is every bit a war of necessity.

"If the international forces were to pull out before the Afghan government was able to provide security, global terrorism would have free rein again.

"Success will come when the enemy is weakened enough for the Afghan army and government to contain it.

"Afghanistan is not a campaign without end. Nor is it one we can walk away from."

Meanwhile Lieutenant General Jim Dutton, the most senior British commander in Afghanistan, told BBC television: "collectively we have not been as good at explaining this as we ought to be."

He said that public support was "crucially important" for the mission.

"There may be those who don't support the policy... (but) I sense public support for what soldiers are doing," he added.

A total of 230 British troops have died in the Afghanistan mission since operations began against the Taliban extremists in October 2001. Of those, at least 199 were killed as a result of hostile action.

Britain's 9,000 troops in Afghanistan, largely battling Taliban insurgents in the troubled southern Helmand Province, are trying to train up local forces to take over security control for themselves.

"We can achieve a level of security within three to four years that allows the Afghans to take over," Dutton said.

"I have to say yes, it is worth some soldiers having to die for because the consequences of it going wrong are far greater.

"All our populations back home will put up with the cost of this sort of operation -- and I mean the cost in human and financial terms -- if they believe two things: one, that we're right, and two, that we can win."

On Tuesday, a "rogue" Afghan policeman shot dead five British troops and wounded others in an attack which underscored the increasing complexity of the war.

The killings heightened concerns as Britain is hoping to shift its troops away from front-line fighting to training such local forces.

The attack was a "monstrous" act of treachery, the commanding officer of some of the murdered men told The Sunday Telegraph newspaper.

But despite the shock and disgust, the incident was probably a "one-off", said Lieutenant Colonel Roly Walker, the commanding officer of the 1st Battalion Grenadier Guards.

"If we were to allow any wedges to form between us, they (the police) would wither and they would fall prey to the Taliban," he added.

"So I have said to the boys: 'it's no change'."

Queen Elizabeth II is set to lead the annual Remembrance Sunday service and parade in central London, paying tribute to the thousands of men and women who died in world conflicts.

This time, alongside the victims of previous wars, Britons must also think about "all those who have been killed in Afghanistan this year -- heroes who have lost their lives on Afghan streets so that we might be safer on Britain's streets," Brown said.

ComRes surveyed 1,009 adults of different ages and social classes across Britain for BBC television's "The Politics Show".