Washington, Dec 2  US President Barack Obama's new strategy for the conflict in Afghanistan has drawn sharp criticism from both opposition Republicans and within the ranks of his own Democrats.

Obama unveiled the plan - anchored by the deployment of 30,000 more troops to the war zone - in a national address Tuesday evening.

The reinforcements are to surge into Afghanistan by July - with a timeframe a year later for the beginning of withdrawals and the transfer of security responsibility to the Kabul government, depending on conditions on the ground.

Obama's strategy seeks to delicately balance the need to prevail in Afghanistan while accommodating the growing scepticism in the US over the prospects for success, with polls showing Americans increasingly opposed to the war.

If his strategy fails, left-wing Democrats can claim more troops should not have been sent in the first place, while Republicans will blame it on setting a target date for withdrawals.

After months of complaining that Obama has moved too slowly in weighing his options, Republicans applauded the force buildup, which will expand the US presence to 98,000 troops. But they hammered Obama for setting a timeframe for beginning pullouts, arguing a date could prompt the Taliban to go underground and wait out the surge.

"Dates for withdrawal are dictated by conditions. The way that you win wars is to break the enemy's will, not to announce dates that you are leaving," said Senator John McCain, Obama's opponent in the 2008 presidential election.

"It may convey the impression that we are going to be there for a short period of time, and the Taliban just have to wait us out."

Former vice president Dick Cheney, who along with Obama predecessor George W. Bush adamantly resisted setting withdrawal deadlines for Iraq, lashed out at the White House for setting a pullout timeframe in Afghanistan.

Cheney said it sends the wrong signal to Afghans, who, by knowing the US plans to leave, may see a long-term benefit in siding with the Taliban.

"Those folks ... begin to look for ways to accommodate their enemies," he said in an interview with Politico. "They're worried the United States isn't going to be there much longer, and the bad guys are."

Obama addressed the criticism during his speech at the US Military Academy at West Point in New York, saying an exit strategy will keep pressure on the international community and Afghan government to achieve results.

"The absence of a timeframe for transition would deny us any sense of urgency in working with the Afghan government," Obama said. "It must be clear that Afghans will have to take responsibility for their security, and that America has no interest in fighting an endless war in Afghanistan."

Despite Republican criticism, Obama can still expect widespread support from the opposition party that has long called for sending more troops to Afghanistan. But the move has divided the president's own Democrats, who control both chambers of Congress.

Moderate Democrats have backed Obama's plan, but those further on the left have been among the biggest critics, preferring withdrawal and arguing that more troops will only further entangle the US in a prolonged quagmire.

Congressional Democrats also worry that with polls showing the public has turned against them and the Afghan war, they could lose their majority in 2010 by-elections.

"I certainly continue - and I think my colleagues continue - to question the wisdom of sending in tens of thousands of more troops into Afghanistan," Senator Russ Feingold of Wisconsin said.

Obama has reached "the wrong conclusion", said Representative Jim McGovern, a Democrat from Massachusetts. "If our fight is truly with Al Qaeda, then we're in the wrong country. They have moved to Pakistan. I've seen this movie before, and it doesn't have a happy ending."

Obama refuted that criticism - explicitly rejecting comparisons to the Vietnam War - saying that to "muddle through" with current troop levels would "permit a slow deterioration".

"It would ultimately prove more costly and prolong our stay in Afghanistan," he said, "because we would never be able to generate the conditions needed to train Afghan security forces and give them the space to take over." (dpa)