The general overseeing US forces in Afghanistan told senators Tuesday that he recommended keeping 13,600 American troops in the country once NATO withdraws in 2014, even as White House officials have pushed for a smaller presence.

It was the first time a senior military leader had revealed his advice on how many troops should stay in Afghanistan as President Barack Obama's administration carries out an internal debate on the size of a future force after 2014.

At a Senate Armed Services Committee hearing, General James Mattis, head of US Central Command, was asked by Senator John McCain what he had recommended on future troop levels and he said: "That recommendation is for 13,600 US forces, sir."

And the American troops would be joined by several thousand non-US NATO forces, with "around 50 percent of what we provide," said Mattis, who as Central Command chief presides over troops in the Middle East and Afghanistan.

His comments confirmed speculation that the military prefers a larger troop presence in Afghanistan after 2014 than some senior White House officials, who have leaked proposals for a follow-on force of 6,000-9,000 boots on the ground.

The Obama administration is anxious to wrap up the unpopular war and to cut costs amid deep budget cuts, but some commanders worry that battlefield gains from a troop "surge" in 2010-11 could unravel without a more robust military presence.

US defense officials said no final decision has been made on how many troops should remain in Afghanistan beyond 2014 and it was unclear if the military's top officer, General Martin Dempsey, endorsed Mattis' advice.

Obama last month announced 34,000 troops would return from Afghanistan by mid-February 2014, reducing the force to about 32,000. At the moment, there are roughly 66,000 troops in the country, after a peak of about 100,000 in 2011.

The White House clashed with top brass during an earlier debate in 2009 over the planned troop reinforcements, with senior military officers pushing for a longer surge of troops with no deadline for withdrawal.

At the hearing, Mattis told senators he believed the war effort was "on track," describing "a combination of progress and violence."

But he insisted Afghan security forces were proving increasingly capable and that casualty figures showed the Kabul government's army and police were now doing the lion's share of the fighting.

"Since the 1st of January, we have lost four US troops, four of our wonderful troops, killed in action," he said.

"In the same period, the Afghan security forces have lost 198 killed. There can be no longer any doubt -- it's not opinion. It's now a fact -- the Afghans are doing the bulk of the fighting, and they are doing it with our support."