Prime Minister Tony Blair defended Britain's presence in Iraq against sharp criticism Wednesday, saying it would be a "gross dereliction of our duty" to withdraw before Iraqi troops are able to take responsibility for security.

The comments came after opposition Conservative Party leader David Cameron pressed the prime minister about the assertion last week by Gen. Richard Dannatt, the head of the army, that British forces should leave Iraq soon because their presence provokes more violence than it prevents.

"I do not want to either dismay our allies or hearten our enemies by suggesting we will do anything else than stay until our job is done," Blair said. Britain's strategy, he said, "is to withdraw progressively as the Iraqi forces build up their capability."

In a heated exchange during Blair's weekly House of Commons question session, Cameron accused the prime minister of presenting Britons with a picture of events in Iraq that is far brighter than the reality.

"Will he give a guarantee of frank, candid and honest answers?" the opposition leader demanded.

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Menzies Campbell, leader of Britain's third-largest party, the Liberal Democrats, said that with violence increasing in Iraq it was clear a new plan was needed.

"The government strategy has failed," he said. "And in those circumstances the choices are stark — change the strategy or else get out."

Blair rejected that argument.

"If we desert the Iraqi government now, at the very time when they are building up their forces ... it would be a gross dereliction of our duty," he said.

"If we got out now, when the job wasn't done, and simply deserted the situation, what good would that do other than to make sure that those people that support these extremists right around the world would take heart from it?" he added.

Blair has struggled to stamp out the furor following Dannatt's statement that the presence of British troops is provoking violence in Iraq, not preventing it.

Dannatt later tried to tone down his remarks — saying he wanted a gradual pullout over the next few years — but he did not back away from them entirely.