France's decision to withdraw combat troops from Afghanistan a year earlier than planned deals a blow to the US-led war effort and threatens to trigger a "rush to the exits" by other NATO members, experts said Monday.

President Nicolas Sarkozy's surprise announcement to pull out his country's forces in 2013 "upends a well-planned, well laid-out NATO strategy of transition in Afghanistan going through 2014," said Damon Wilson, executive vice president of the Atlantic Council and former White House official.

At a meeting of defense ministers this week in Brussels, an anxious United States will be engaging in a round of "damage control" to shore up the alliance's agreed upon timeline, Wilson told reporters.

"You're going to look for the US trying to have as many defense ministers to their press conferences, restating their commitment to the NATO strategy" for a 2014 withdrawal, he told an event organized by the Atlantic Council think tank.

Days after the killing of four French troops by a renegade Afghan soldier, Sarkozy said France's combat forces would leave in 2013 and called on other NATO states to shift to an earlier timetable.

Sarkozy's abrupt change in course last week represents a "stunning" unilateral move that poses "a real challenge to NATO leadership and (President Barack) Obama's leadership," said Ian Brzezinski, a former Pentagon official under George W. Bush's presidency.

Sarkozy had done much to repair France's ties with Washington and NATO in the wake of a bitter falling out over the Iraq war, winning praise for his stance on Afghanistan, Iran's nuclear program and his leading role in the NATO-led air war in Libya.

But the decision to speed up the French military withdrawal from Afghanistan despite a NATO consensus is seen as a betrayal in Washington and London and will damage Paris's credibility in the alliance, analysts and former officials said.

"One of Sarkozy's most prominent achievements has been to rehabilitate France's reputation in London and in Washington, that France can be a trusted ally, that it's taking its place within NATO," said Jeffrey Lightfoot, deputy director of Atlantic Council's International Security Program.

For the Taliban insurgency, France's early exit is "a smashing propaganda victory" that encourages targeting European troops to damage a "fragile political consensus" within the coalition, Lightfoot said.

The right-leaning Wall Street Journal portrayed Sarkozy as bowing to domestic political expediency, but argued the Obama administration had opened the door for a mass exodus by setting a withdrawal deadline in the first place.

"Still, it would be unfair to lay too much blame on Mr. Sarkozy, who is only trying to get ahead of the coming stampede for the exits," the Journal wrote in an editorial.

"That was bound to happen the moment President Obama announced a timetable for the surge and a date-certain for withdrawal, thereby giving the Taliban hope that they could bide their time while giving America's coalition partners no good reason to stay."

The pace of the French troop drawdown will play a crucial role in how other coalition partners react, Michael O'Hanlon of the Brookings Institution, told AFP.

"The key thing for me is how fast France downsizes in 2013, which is shaping up as a crucial year. If most of the current forces stay through 2013, I think it's not a huge problem," he said.

But if the bulk of 3,600-strong French contingent withdraws next year, NATO's scheduled 2014 drawdown could unravel, he said.