WASHINGTON (Reuters) - Top U.S. officials said the first of 30,000 new U.S. troops will arrive in Afghanistan in two to three weeks, but also made clear on Wednesday that plans to start bringing the soldiers home in 18 months could slip.

One day after President Barack Obama unveiled his high-risk strategy for the Afghan troop surge, Defense Secretary Robert Gates and Secretary of State Hillary Clinton faced skepticism from lawmakers of a big escalation of the unpopular and expensive war.

“This is a huge commitment. It’s the right commitment. And it gives us the forces to turn this thing around,” Admiral Mike Mullen, chairman of the military Joint Chiefs of Staff, told the House of Representatives Foreign Affairs Committee.

Obama’s plan will bring the U.S. troop strength in Afghanistan to almost 100,000 in a buildup officials hope will secure the country after eight years of war and allow U.S. soldiers to start pulling out by the summer of 2011.

Defense Secretary Robert Gates, testifying at the Senate Armed Services Committee, said the first new U.S. forces would be sent in 2-3 weeks, starting an 18-24 month “extended surge”.

Gates said the aim was to shift responsibility for security to the Afghans themselves. “Beginning to transfer security responsibility to the Afghans in summer 2011 is critical - and, in my view achievable,” he said.

But U.S. commanders were keeping options open on the timing of a troop drawdown. Gates said they would review progress in December 2010 and final decisions on when to begin the withdrawal would depend on that assessment.

He said Washington would not abandon Afghanistan if the security situation appeared untenable. “We’re not just going to throw these guys into the swimming pool and walk away,” he said.

Obama acknowledged that Americans were war-weary after the bloody, six-year conflict in Iraq. But he depicted the Afghan campaign as vital to U.S. security and said it was aimed at defeating the Taliban and preventing further attacks by al Qaeda, which was behind the September 11, 2001 attacks.

The top Republican on the Senate committee, John McCain, voiced doubt about the withdrawal plan, echoing fears that it could allow Taliban militants to wait out the U.S. troop surge and reassert themselves later.

“A date for withdrawal sends exactly the wrong message to both our friends and our enemies,” said McCain, Obama’s defeated Republican rival in the 2008 presidential election.

OBAMA’S WAR

Obama’s speech made him the architect of a new phase of the eight-year old Afghan war, adding about $30 billion in costs in the coming year as the country struggles with record federal deficits, high joblessness and the on-going economic bailout.

Many of Obama’s fellow Democrats have voiced doubt about escalating the costly conflict, while Republicans have complained that the drawdown date ties the military’s hands.

The debate carries clear risks for Democrats ahead of mid-term elections next year in which Republicans hope to take chunks out of Democratic majorities in both chambers of Congress. Obama himself faces re-election in 2012.

“Like everything else, it depends on how it goes,” Democratic Senator Paul Kirk, a former chairman of his party, said of Obama’s Afghan strategy. “There were absolutely no good options.”

Mullen said 20,000 to 25,000 more U.S. troops would be in Afghanistan by the height of the summer fighting season in July and final deployments could take place as late as the fall.

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But plans to pull them out -- which could prove politically explosive -- were less clear.

Under questioning by Republicans, Gates and other top officials suggested the 18-month timeline on withdrawals could change if circumstances show the fight is not being won.

“I do not believe we have locked ourselves into leaving,” Secretary of State Hillary Clinton told the committee.

Mullen said he did not anticipate any request for more troops, while Gates dismissed suggestions the United States could get stuck in a Vietnam-style quagmire.

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“If I came to the conclusion that we were bogged down and stalemated, and we were sending young men and women into a maw with no purpose and no hope of success, I wouldn’t sign any more of those (deployment) orders,” he said.

PAKISTAN, AFGHANISTAN UNDER PRESSURE

Clinton said Pakistan -- Afghanistan’s fragile, nuclear-armed neighbor -- would be pressed to more actively pursue militants in its own territory and promised that Washington would press Afghan President Hamid Karzai to fulfill promises to fight corruption.

Karzai’s office issued a statement that said Afghanistan welcomed Obama’s change in strategy, although unusually it did not provide a comment from Karzai himself, often depicted as a weak link in Obama’s Afghan strategy.

The top U.S. and NATO commander General Stanley McChrystal, who had said he needed as many as 40,000 troops to win the war, welcomed Obama’s promise to boost forces.

But the Taliban, in a statement issued by email, said the increase would only bolster their resolve. “This strategy by the enemy will not benefit them,” it said.

European leaders were quick to voice support for the new U.S. plan, but most delayed committing new troops to an uncertain, unpopular and deadly military campaign. U.S. officials have said Washington in seeking 5,000-7,000 more troops from allies.

Britain promised to send 500 extra soldiers, boosting its contingent to about 10,000. Poland said it would send 600 more to join its 2,000-strong force while Italy promised an unspecified number. Others kept it vague, reflecting widespread unease with a war grown increasingly unpopular with many western voters.