WASHINGTON (Reuters) - The United States should be able to begin scaling back its troop presence in Afghanistan from a post-surge peak by 2013, according to an assessment by the top U.S. and NATO commander in Afghanistan.

U.S. Army General Stanley McChrystal gave his assessment to a delegation of U.S. lawmakers who visited Kabul earlier this week, participants said on Saturday.

It is unclear whether President Barack Obama will spell out a schedule for a drawdown of U.S. forces when he announces a revised war strategy on Tuesday.

The centerpiece of that strategy is expected to be the gradual deployment of about 30,000 more U.S. troops to Afghanistan to secure population centers and train more of the country’s security forces.

Pentagon officials hope NATO members will contribute up to 10,000 extra troops and trainers to supplement the U.S. buildup, pushing the overall number closer to the 40,000 additional soldiers recommended by McChrystal to counter a resurgent Taliban.

Britain has said it expects countries to pledge another 5,000 troops on top of those sent by the United States.

A drawdown schedule could help Obama blunt opposition to a troop increase within his own Democratic party, which controls Congress. Rising U.S. casualties and military costs have sapped public support for the war, threatening Obama’s standing ahead of congressional elections next year.

An international conference on Afghanistan, to be held in London in January, would aim to set conditions for a gradual transfer of security responsibilities to Afghan control.

McChrystal met the visiting U.S. lawmakers on Tuesday.

“I asked him: ‘If you get these troops that you are requesting, the 40,000, where’s the tipping point? At what point will we begin to draw down?’” Republican Representative Mike Coffman told Reuters. “McChrystal responded: ‘Sometime before 2013.’”

Pentagon officials had no immediate comment.

There are now roughly 68,000 U.S. troops and 42,000 allied forces in Afghanistan.

Coffman said McChrystal’s assessment that a drawdown could begin before 2013 was “realistic” given the difficulty of training enough Afghan army and police units to take over security responsibility from U.S. and NATO troops.

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“Irrespective of that, any increase in troops is going to be a challenge in Congress,” Coffman said, citing Democratic opposition.

One proposal made by leading Democrats would set the goal of having about 400,000 Afghan army troops and national police officers by 2012 -- more than twice the forces’ current size and a year earlier than currently planned.

But some administration officials questioned whether that was realistic.

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Defense Secretary Robert Gates has said an eventual drawdown in Afghanistan could follow the Iraq model, under which U.S. forces could pull back from city centers as Afghan forces take the lead.

As an interim step, officials said, a transition to greater Afghan control could begin within the next year in parts of Afghanistan that are more stable.

McChrystal’s target for starting a drawdown would fall within the five-year timeframe cited by Afghan President Hamid Karzai for when he sees his forces being ready to take over from Western troops.

The White House has said it sees U.S. troops out of the country within the next eight to nine years. But the Pentagon has cautioned against setting specific dates, saying any handover depends on conditions on the ground.