BRUSSELS — Germany and Italy will join the United States as "lead nations" in regions of Afghanistan after NATO transitions into a noncombat mission there after 2014, Defense Secretary Chuck Hagel said Wednesday.

The U.S. defense chief was speaking after the Atlantic alliance laid out a new plan shifting into a training and assistance role for the Afghan forces set to take over from NATO in about 18 months. He didn't specify, however, how many troops NATO will maintain in Afghanistan after that.

"The United States has committed to being the largest single contributor to this mission" — as lead nation in the restive east and south, Hagel told reporters after a two-day meeting of NATO defense ministers at the alliance's headquarters in Brussels.

"We appreciate the commitments that other nations are making, including announcements by Germany and Italy that they will serve as lead nations for the north and the west" — zones the two European are in charge of now in the fight against insurgents in Afghanistan.

Turkey, he said, was "favorably considering" a role as "framework nation" in Kabul, the Afghan capital.

Questions remain about the total size of the NATO noncombat mission force after the end of next year.

NATO Secretary-General Anders Fogh Rasmussen said only that it will be "significantly smaller" than the tens of thousands of U.S.-led troops in Afghanistan now, and said decisions about force numbers after 2014 will come later.

There currently are about 100,000 international troops in Afghanistan, including 66,000 from the United States. The U.S. troop total is scheduled to drop to about 32,000 by early next year, with the bulk of the decline occurring during the winter.

The new mission "will not be ISAF by another name," Fogh Rasmussen said of NATO's International Security Assistance Force in Afghanistan. "It will focus on the national institutions such as the security ministries, and the core level of army and police command."

There has been sharp debate over whether the United States should be more specific about its long-term military commitment to Afghanistan. Some defense experts have suggested that NATO's plan for a residual force of 8,000 to 12,000 may not be large enough.

Last week, retired Gen. John Allen and a former undersecretary of defense urged the White House to announce its plan as soon as possible.

Training Afghan military and police to take over security is essential to the withdrawal of NATO combat forces remaining in Afghanistan nearly 12 years after toppling the Taliban's hardline regime for sheltering al-Qaida's terrorist leadership.

The start of the insurgents' spring fighting season in April has been a crucial test for those forces, as U.S. military trainers pull back.