WEST POINT, New York - President Barack Obama on Tuesday announced an immediate 30,000 strong-troop surge for Afghanistan, vowing to "seize the initiative" to finally end the unpopular and costly eight-year war.

But in a major speech unveiling a new fast-track war strategy, Obama said American forces would start coming home in 19 months time, as he groped for an exit from a conflict many backers see as a Vietnam-style quagmire.

"Afghanistan is not lost, but for several years, it has moved backwards," Obama said, placing a bet that more forces could defeat Al-Qaeda, crush a resurgent Taliban and pave an elusive route home for US forces.

The speech, before young cadets at the US Military Academy at West Point who will fight Obama's war, marked his biggest test yet as president, and best opportunity to redefine the conflict.

For the first time, the president established a certain date for the start of a US withdrawal from Afghanistan - but stopped short of establishing a deadline to complete a mission launched after the September 11 attacks in 2001.

"As commander in chief, I have determined that it is in our vital national interests to send an additional 30,000 troops to Afghanistan," Obama said.

"After 18 months, our troops will begin to come home," he said, seeking to reassure Americans the new plan, which will cost an extra 30 billion this year alone, would not signal a war without end.

The first US Marines could be in place by Christmas, just four months after war commander General Stanley McChrystal warned that the war could be lost without more manpower.

All the new troops will be fighting within six months. Obama also cranked up pressure on NATO allies for more troops, saying they were also threatened by Afghan-based terrorism, and declaring the battle was a defining test of credibility for the western alliance.

Following an exhaustive strategy review - dismissed by Republicans as "dithering" Obama rolled out new political approaches to Afghanistan and Pakistan and vowed to chase down Al-Qaeda wherever it emerged.

He warned the Afghan government of President Hamid Karzai, that the days of a US "blank check" were over, demanding a drive against corruption.

The White House said a key element of the political strategy would be backing for Afghan efforts to reintegrate Taliban member who renounce Al-Qaeda, lay down their arms and enter politics.

The same "cancer" of extremism that had torn at Afghanistan, was also hurting Pakistan, Obama warned, crediting Islamabad with an unprecedented effort to combat home-grown extremism.

"We will act with the full recognition that our success in Afghanistan is inextricably linked to our partnership with Pakistan," Obama said, promising to forge "mutual interests, mutual respect and mutual trust" with Islamabad.

In a speech targeting multiple audiences at home and abroad, Obama also rejected dire parallels between the Afghan war which haunts his presidency, and Vietnam as a "false reading of history."

"Unlike Vietnam, we are joined by a broad coalition of 43 nations that recognizes the legitimacy of our action. Unlike Vietnam, we are not facing a broad-based popular insurgency," Obama said.

In simple, direct language, shorn of the rhetorical flourishes normally seen in his soaring speeches, Obama said he understood the human and financial costs of the war, but saw not choice but to escalate it.

Obama also vowed to "keep the pressure on Al-Qaeda" which used a safe-haven in Afghanistan to plot the September 11 attacks.

He cautioned that strikes against America were "being plotted as I speak," and said he would go after Osama bin Laden's terror group in Somalia or Yemen or further afield if necessary.

As many of Obama's Democratic allies balk at the cost of the new troop surge, Obama warned the new Afghan war operations would cost 30 billion dollars this year.

But in a swipe at the former Bush administration, blamed for not paying for conflicts in Afghanistan and Iraq, he said he would be open and honest in financing the operation - though he did not offer any firm commitments.

Republican Senate Minority Leader Mitch McConnell, one of Obama's fiercest critics in the US Congress, praised the new "surge of forces" and said it would help "reverse the momentum of the Taliban."

But some anti-escalation Democrats were already plotting to block the surge.

In a statement issued from Kabul, McChrystal praised his boss for setting him a "clear mission and the resources to accomplish our task."