WASHINGTON - President Barack Obama's top deputies on Wednesday defended his plan for a rapid ramp-up in Afghanistan, but some U.S. senators questioned a proposal to set an 18-month timeline for starting to bring troops home.

Obama on Tuesday announced he was sending 30,000 more U.S. troops to fight the Taliban, boosting overall U.S. troop presence to almost 100,000 in a surge that 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 key officials at the Senate Armed Services committee, said the first of the new U.S. troops could hit the ground in Afghanistan in 2-3 weeks, the initial wave of a quick buildup with a definite end-point.

Beginning to transfer security responsibility to the Afghans in summer 2011 is critical - and, in my view achievable, he said.

But the top Republican on the Senate committee, John McCain, raised doubts about the withdrawal plan -- echoing widespread 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 he believed all Americans should support Obama's overall effort to stabilize Afghanistan and eliminate safe havens for al Qaeda extremists..

What I don't support, and what concerns me greatly, is the president's decision to set an arbitrary date beginning the withdrawal ...(a) date for withdrawal sends exactly the wrong message to both our friends and our enemies, he said.

U.S. Army Gen. David Petraeus told MSNBC that the 18-month timeline 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, he said.

OBAMA'S WAR

Obama speech set him up as the architect of a new phase of the Afghan war that could cost up to $30 billion in the coming year as the United States struggles with record federal deficits and the economic bailout.

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

Gates, under sharp questioning by McCain, said U.S. commanders would assess late next year how far along they were in the effort to turn the war over to Afghan forces.

Our current plan is we will begin the transition in local areas in July of 2011. We will evaluate in December 2010 whether we believe we will be able to meet that objective, he said, adding later that the U.S. would not leave the Afghans to fend for themselves if they were not prepared..

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

Secretary of State Hillary Clinton, who joined Gates and Joint Chiefs Chairman Mike Mullen giving testimony on Wednesday, said Washington would press Afghan President Hamid Karzai -- widely seen as a weak link in the strategy -- to deliver on promises to fight corruption.

She also said she supported an Afghan-led effort to bring in moderate elements of the Taliban which renounce violence and said that -- after three months of painful and sometimes public debate over Afghanistan -- Obama's team was now agreed.

There is absolute unity, she said.

KARZAI MUM

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 pledge to significantly 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.

With the first U.S. troops due to arrive in just weeks, all 30,000 should be in place by the end of August at the height of the summer fighting season.

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

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)

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