Obama last week outlined a plan to withdraw all but 9,800 American troops from Afghanistan by the end of this year and to pull out the rest by the end of 2016, ending a more than decade-old combat role triggered by the Sept. 11 attacks on the United States.

The plan foresees a cut in the U.S. troop presence to about half of the 9,800 number by the end of 2015, when U.S. forces will pull back from provincial bases to Kabul and Bagram, the largest U.S. base to the north of the capital.

Defense ministers from the 28 NATO nations, meeting in Brussels on Tuesday and Wednesday, will discuss the implications of the U.S. timetable for NATO's plans to launch a new mission, dubbed "Resolute Support", next year to train and advise Afghan forces after most NATO combat troops leave by the end of 2014.

Several NATO diplomats said the U.S. timetable raises questions about whether the alliance's plan to train the Afghan army from regional bases around Afghanistan can last for more than one year, given the reduced U.S. presence after that.

U.S. ambassador to NATO Douglas Lute told a news briefing it was unclear if the NATO mission would last beyond 2015, even assuming the next Afghan president signed agreements with the United States and NATO on the legal basis for their presence.

"I would say that the announcement out of Washington … assured ... that we will have 'Resolute Support' for at least a year," he said.

"Now what follows that is going to depend on planning that hasn’t been done yet. So I can't state beyond one year, but I think the U.S. contribution solidifies 'Resolute Support' for the first year," he said.

A senior military representative from a NATO ally said he expected other NATO members to contribute 3,000-4,000 soldiers on top of the U.S. commitment after 2014.

Under NATO's plan for its new training mission, Turkey has agreed to be the lead nation in Kabul, Germany in the northern town of Mazar-e-Sharif, Italy in the western town of Herat and the United States in the south and east.

CHANGE OF PLAN

But one senior NATO diplomat said the U.S. troop reduction decision meant the planned NATO scheme of a hub in Kabul and four regional "spokes" would not be sustainable after 2015.

"It will be a slightly different model for 2016 and by the end of 2016 it will no longer really be a training, advise and assist mission at all," he said, speaking on condition of anonymity.

The U.S. announcement raises questions over whether Germany and Italy will be prepared to continue to run the training and advisory mission from the north and west of Afghanistan after the United States scales down its presence by the end of 2015.

If they are not, the scope of the NATO training mission may shrink after 2015 and focus on mentoring Afghan officials in Kabul.

"As the U.S. presence draws down, allies will have to make a judgment about what that means for them, for example, in the north in Mazar, or in the west in Herat in particular, and what their presence will look like," Lute said.

A key issue for other NATO allies is "enablers" - support forces such as strike aircraft and transport and refueling planes - the bulk of which have been provided by the United States.

Lute said the United States remained committed to contributing 'enablers', but added: "That doesn’t mean that the U.S. is going to provide resources for every single allied need in every single location. Because to some extent we view these as national obligations to support their own troops."

The 9,800 troops U.S. troops that remain next year will be split between the NATO-led training mission and a U.S. counter-terrorism mission against remaining al Qaeda targets.

U.S. Defense Secretary Chuck Hagel told reporters traveling with him to Brussels that how these troops would be split up was to be decided and would depend on commitments of allies.

"Obviously we want to work with NATO/ISAF partners on getting their commitments on more specifics on what they want to contribute; numbers of troops, kinds of troops," he said, referring to the NATO-led International Security Assistance Force. "Then we’ll make some decision on assignments."