WASHINGTON - The first of 30,000 new U.S. troops will arrive in Afghanistan in two to three weeks, top U.S. officials said on Wednesday, even as they made clear plans to start bringing the soldiers home in 18 months could slip.

President Barack Obama on Tuesday announced the major increase in U.S. forces fighting the Taliban, bringing the U.S. troop presence there to almost 100,000 in a buildup officials hope will secure Afghanistan and allow U.S. soldiers to start pulling out by the summer of 2011.

Defense Secretary Robert Gates, leading off testimony by top Obama officials at the Senate Armed Services Committee, said the first of the new U.S. forces would be sent in 2-3 weeks, starting a quick buildup with an end-point.

Gates said the aim was to start shifting responsibility for security to the Afghans themselves as soon as possible. Beginning to transfer security responsibility to the Afghans in summer 2011 is critical - and, in my view achievable, he said.

But in a sign that U.S. commanders were keeping their options open, Gates said they would review progress in December 2010 and would not abandon Afghanistan to its fate if the security situation was untenable.

We're not just going to throw these guys into the swimming pool and walk away, he said.

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.

McCain, Obama's defeated Republican rival in the 2008 presidential election, said the goal of securing Afghanistan and eliminating safe havens for al Qaeda extremists was admirable, but an arbitrary U.S. pullout date was dangerous.

A date for withdrawal sends exactly the wrong message to both our friends and our enemies, he said.

U.S. Army General David Petraeus, credited with using a similar troop surge and counter-insurgency strategy to pull Iraq back from the brink, told MSNBC the 18-month withdrawal timeline for Afghanistan was realistic but ambitious.

It will be very challenging. There will be nothing easy about it. There has been nothing easy. Afghanistan is hard and it's hard all the time and we have our eyes wide open about that, said Petraeus, who now commands U.S. forces in the whole region.

OBAMA'S WAR

Obama's speech set him up as the architect of a new phase of the eight-year old Afghan war, adding $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 comes at a risky time for Democrats, with an anti-incumbent mood building ahead of mid-term elections next year that could see Republicans take chunks out of Democratic majorities in both chambers of Congress.

Obama himself is up for re-election in 2012.
Gates and other top officials, under sharp questioning by Republicans, suggested that the 18-month withdrawal timeline could change if circumstances on the ground indicate the fight is not being won.

I do not believe we have locked ourselves into leaving, Secretary of State Hillary Clinton told the committee, saying the United States wanted to show it did not intend to occupy Afghanistan indefinitely.

We're not interested in running their country, building their nation. We are trying to give them the space and time to be able to build up sufficient forces to defend themselves, she said.

Clinton said Washington would press Afghan President Hamid Karzai to deliver on promises to fight corruption and would support moves to bring in moderate elements of the Taliban which renounce violence.

KARZAI'S OFFICE WELCOMES PLAN

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.

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, said Obama's promise to boost forces was the end of the beginning of the conflict.

But the Taliban, in a statement issued by email, said the increase would only increase 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 was first off the mark, promising 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.

Germany, France and others said they would wait and see.

NATO Secretary-General Anders Fogh Rasmussen, speaking in Brussels, said it was realistic to expect Afghans to take over security work in 10 to 15 areas and districts next year but the transition could only happen if conditions were met.

We will not leave unless we feel sure the Afghan security forces can actually take on responsibility for that specific district or province, he said.

Mullen said the Taliban was now dominant in 11 of Afghanistan's 34 provinces but that the troop buildup would give the U.S. military the upper hand.

Marking a major shift in strategy, McChrystal said the vast majority of the new combat troops would be fielded in partnership with Afghan units, a counter-insurgency mentoring tactic he said had not been fully possible in the past because the Afghan army and police were too small.

(Additional reporting by Adam Entous, David Morgan, Vicki Allen and Mohammad Zargham in Washington; David Brunnstrom in Brussels, Sayed Salahuddin in Kabul and Zeeshan Haider, Augustine Anthony and Michael Georgy in Islamabad; Editing by David Storey)