US President Barack Obama's national security team heads to Congress on Wednesday to address concerns from skeptical lawmakers on expanding the war in Afghanistan by sending another 30,000 troops.

Rising combat deaths and military costs have sapped public support for the eight-year-old war and Obama's troop increase has prompted protests from left-leaning leaders of his Democratic Party ahead of next year's congressional elections.

In his televised speech on Tuesday, Obama said the goal of raising US troop levels to nearly 100,000 was to step up the battle against the Taliban, secure key centers and train Afghan forces so they can take over, allowing for a US exit.

Allies were also expected to send more soldiers, with Obama saying "the common security of the world" was at stake.

Obama's pledge to start bringing US troops home after 18 months, provided conditions on the ground allow it, may help him to contain rebellion among Democrats.

But the July 2011 drawdown date drew swift condemnation from Republicans who, while critical of Obama's three-month strategy review, were nonetheless behind his final decision to bolster the 68,000 US troops now in the war zone.

"All Americans want to see our troops leave Afghanistan as soon as possible after successfully completing their mission," said Representative Howard McKeon, the senior Republican on the House of Representatives Armed Services Committee.

"But we want that redeployment to be based on the events and conditions on the ground -- not the Washington political clock."

Republicans argue that setting withdrawal timelines only emboldens the Taliban and undermines support for US-backed governments in Afghanistan and Pakistan.

Key congressional committees scheduled back-to-back hearings on Wednesday and Thursday to review Obama's revised war strategy, estimated to cost $30 billion this fiscal year.

As commander-in-chief, Obama has the authority to send the soldiers but Congress must approve the cost.

Secretary of State Hillary Clinton, Defense Secretary Robert Gates and the chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, will appear first before the Senate Armed Services Committee at 9 a.m. EST (1400 GMT), followed by the House Foreign Affairs Committee at 1:30 p.m. (1830 GMT).

Army General Stanley McChrystal, the commander of US and NATO forces in Afghanistan who had recommended sending 40,000 more troops, will testify next week.

Gates will describe "the symbiotic relationship between al Qaeda and the Taliban and the urgent need to reverse their momentum in both Afghanistan and Pakistan," said Pentagon press secretary Geoff Morrell.

"He will candidly discuss the mistakes we have made in that region over the last few decades and the imperative that we not repeat them," Morrell said.

Shorter timeline

Major US troop movements are likely to begin in January and all 30,000 troops should be in place by the end of August, a far shorter timeline than war planners had expected.

Other NATO members are expected to commit between 5,000 and 7,000 additional troops, although some of them are already deployed as part of the alliance's 42,000-strong contingent.

Key Democrats welcomed Obama's decision to set a date to begin a drawdown but reserved judgment on the troop increase.

"President Obama asked for time to make his decision on a new policy in Afghanistan. I am going to take some time to think through the proposal he presented," said Democratic Senator Dick Durbin, from Obama's home state of Illinois.

In both parties, mounting war costs have taken center stage because of unease about the US national debt of nearly $12 trillion and demands on the government from help for the struggling economy and a proposed healthcare overhaul.

"The cost of this is astronomical," said Republican Representative Walter Jones.

Representative Dave Obey, who chairs the House committee in charge of approving government spending, said a long-term commitment to the Afghan war could cost $500 billion to $900 billion over the next decade, which could "devour our ability to pay for the actions necessary to rebuild our own economy."

Obey, a Democrat, has proposed a war surtax to pay for the conflict. But a tax increase is unlikely, especially with midterm congressional elections next year. – Reuters