The United States hopes to shift its military role in Afghanistan from combat to training during the second half of 2013, Defense Secretary Leon Panetta said on Wednesday.

But he said the US administration wanted to see all the NATO allies in Afghanistan -- including France -- "respect" a strategy adopted at a summit in Lisbon in November 2010, which calls for handing over security duties to Afghan forces by the end of 2014.

"Hopefully by the mid-to-latter part of 2013, we'll be able to make a transition from a combat role to a train and advise and assist role, which is basically fulfilling what Lisbon was all about," Panetta told reporters aboard his plane en route to Brussels.

"We all went in here together and we'll all go out together, but we have to do it on the basis of a strong alliance and a strong commitment that was made in Lisbon," said Panetta, who was due to meet NATO defense ministers in his visit to Belgium.

The NATO-led coalition this year needed to cement battlefield gains against the Taliban insurgency and to build on progress in strengthening the Afghan army and police, he said.

He said 2013 would be a "crucial" year for the final transfer of remaining areas to Afghan security forces.

"2014 becomes a year of consolidating the transition," he said.

Panetta's comments reaffirmed signals from President Barack Obama's administration that after a decade of war, America's military mission in Afghanistan will soon evolve into a supporting role with Afghan forces taking the lead in the fight against the Taliban.

It was unclear how the planned shift from combat to a mainly advisory role would affect planned troop levels for US forces.

With nearly 90,000 US troops now in Afghanistan, Panetta said that "no decision has been made with regards to the level of forces we'll have in 2013."

By the end of September, the number of US troops is due to drop to 68,000, following the scheduled withdrawal of a "surge force" that deployed in 2010.

The Pentagon chief sought to play down the effect of last month's surprise announcement from French President Nicolas Sarkozy to withdraw French combat forces in 2013, a year earlier than planned under the NATO strategy.

"With regards to France, I understand why they made their decision," Panetta said.

Despite the French withdrawal plans, he said he was "pleased" that France had indicated it would retain a longer-term military presence with troops training and advising Afghan forces.

"My hope is at that at this ministerial (in Brussels), we can discuss their decision and hopefully find a way to make sure we bring them back into the Lisbon strategy," he said.

A senior US defense official told reporters it was possible that there was no serious gap between the French stance and NATO's timeline, depending on the precise details of what Paris planned.

"I think the discussions will reveal whether there's a serious difference or not," said the official, who spoke on condition of anonymity.

"What we need to discuss with the French is exactly what role they envisage playing in 2013 and 2014, whether there's a serious difference in terms of the milestones that they envisage," the official said.