"But ... the Afghan people have got to sign off. They've got to be comfortable. They've got to be the ones inviting us to stay as a partner," the Pentagon chief while en route to NATO headquarters in Brussels.
Earlier this month, Secretary of State John Kerry and Afghan President Hamid Karzai reached a preliminary deal for postwar Afghanistan.
The two men "succeeded in defining exactly what the limits would be for American participation" in the country after the White House's 2014 withdrawal deadline, Kerry said in an Oct. 17 interview with NPR.
"Everything that will be necessary to a [postwar] agreement is in the agreement," Kerry said of the preliminary deal reached with Karzai's government.
But the issue of immunity for U.S. forces in country after the 2014 deadline remained unanswered in the preliminary deal.
Lack of immunity for U.S. troops was a crucial factor in the failed attempt to set up a postwar security deal in Iraq, and it set the stage for the recent wave of sectarian violence against Iraqi forces and civilians in the country.
Karzai is set to meet with the Loya Jirga, an assembly of the country's most powerful tribal leaders, in the coming weeks to seek their approval of the postwar plan.
On Monday, Hagel expressed support for the jirga weighing in on the deal, particularly on the issue of troop immunity.
The jirga's ruling "is important because everybody has to be part of this and certainly Afghanistan is a sovereign nation," Hagel said.
"They have to have a very intense role in this," he added.
But the Pentagon chief did make clear that time is running out for the United States, NATO and Afghanistan to come to terms on the immunity issue.
U.S. and allied forces officially handed over control of combat operations to Afghan National Security Forces in July, as part of the White House's plan to have all combat troops out of the country by April 2014.
"We are no longer in any lead positions, no combat roles. So this transition has been underway. It needs to be formalized" via an agreed postwar plan.
Kerry predicted Afghanistan's leaders would ultimately accept a U.S. postwar deal that includes immunity from prosecution for American troops.
"I believe they understand that this agreement is in the interests of Afghanistan because it's an agreement that provides for international support, not just the United States," Kerry said
"They have a choice: Either that's the way it is, or there won't be any forces there of any kind," he added.