BRUSSELS, Belgium - World diplomats issued a stern warning Tuesday to Afghan leaders in a new effort to help secure the war-torn nation's future with thousands of foreign forces after 2014. But officials backed off earlier U.S. threats to withdraw all troops if Afghan President Hamid Karzai does not agree to the offer before the end of the year.

NATO officials said they need a decision, and soon, on continuing a military training mission in Afghanistan through next year. Without clarity, the estimated 50 nations who said they would help transition Afghanistan from 13 years of war will have to start planning how, and when, to send soldiers home.

Secretary of State John Kerry and NATO Secretary-General Anders Fogh Rasmussen resisted holding Karzai to a set deadline for a decision. It was a step back from a White House warning last month that the United States would pull out troops if Afghanistan did not sign a security accord by the end of this year.

"This is not fooling around; this is serious business," Kerry told reporters at the annual NATO meeting in Brussels.

'Move forward'

"I think that it is important for the agreement to try to move forward," he said. "That is what we need to aim for, sooner not later, because that is what is best for Afghanistan."

Karzai has tentatively endorsed the deal, but he shocked allies last month when he refused to sign it after it was approved by a council of tribal elders. The council said the agreement with the United States should be signed by the end of December, as Washington demands.

Instead, Karzai maintains the decision should be left to his successor after elections in April. He has indicated he will not sign any agreement that allows for continued air strikes and foreign raids on Afghan homes. Civilian deaths at the hands of U.S. and allied soldiers have been a source of contention, exacerbated last week by a U.S. drone strike that killed a child.

Awkward

The latest round of pressure on Karzai puts the United States and its NATO allies in the awkward political position of threatening to leave even as they make a case to stay.

Rasmussen, a Dane, said he could not rule out a so-called zero option - the complete withdrawal of foreign forces at the end of 2014. He said it "can be the unfortunate outcome of the decisions - or nondecisions - in Afghanistan."

"We are prepared to stay in Afghanistan beyond 2014 with permission to train, assist, advise the Afghan security forces," Rasmussen told reporters. "But it's no surprise that we had to make clear that we need a firm legal framework for our presence in Afghanistan."