NATO defense ministers agreed here Thursday to begin preparing for a complete withdrawal from Afghanistan at the end of this year, as a senior U.S. military official warned that “the progress we’ve made is not sustainable” by Afghan forces without an ongoing U.S. and international troop presence.

The alliance also expressed strong concern about the tumult in Ukraine and urged Russia “not to take any action that could create misunderstanding.”

NATO’s action on Afghanistan followed President Obama’s order Tuesday to the U.S. military to develop plans for a “zero option” of complete withdrawal if the Afghan government continues to refuse to sign a bilateral security agreement with the United States.

NATO and U.S. defense officials, including Defense Secretary Chuck Hagel, have made clear that their expectation — and their strong preference — is that U.S. and NATO forces will remain in Afghanistan for training and counterterrorism missions after the last international combat troops leave in December.

A twofold decision taken by NATO on Thursday, Secretary General Anders Fogh Rasmussen said, reaffirms the alliance’s commitment to a training mission after 2014. “On the other hand,” he said, “we also had to ask our military authorities to plan for alternatives, because we don’t know yet whether an appropriate legal framework will remain in force or whether we will get an invitation.”

Although both Obama and NATO agree that Afghan President Hamid Karzai does not intend to sign the agreement — originally scheduled for completion last fall — many here chose to look on the bright side and interpreted the U.S. announcement as clarifying a willingness to wait until after Karzai’s successor is elected this spring.

The senior military official cited “great fears” inside Afghanistan that the United States “wouldn’t wait for another administration.” The official said it could be argued that Obama’s statement indicates “a willingness to deal with the next administration. That was never said before.”

While Obama’s order can be interpreted as adding “certainty or uncertainty,” the official said, “I will emphasize certainty” in conversations with Afghans.

Without the pact, the official said, “I think that what we’ll see is deteriorating security conditions over time” as Taliban and other forces regroup to combat Afghan forces. The official spoke on the condition of anonymity to discuss internal U.S. and NATO deliberations.

Gen. Joseph F. Dunford Jr., the commander of coalition troops in Afghanistan, briefed ministers here Wednesday and has drawn up plans that anticipate leaving in place a force of at least 12,000 U.S. and international troops. That “core,” the military official said, will be in place by July and will stay as the rest of what is now a 47,000-strong international force withdraws by the end of the year.

In what is designed as a “hub and spoke” system, the core troops — two-thirds of them Americans — would be based at the Afghan military’s six corps headquarters across the country: in Gardez, Kandahar, Herat, Mazar-e Sharif, Lashkar Gah and Kabul. Bagram would remain an international base with an Afghan military component.

Until this week, the official said, that was “the only plan we were focused on.” The White House is also considering the possibility of a small number of U.S. troops who would be restricted to Kabul and Bagram.

The current plan sets several priorities for the post-2014 mission: building institutional capacity at Afghanistan’s defense and interior ministries, improving Afghan intelligence and aviation capabilities, and continuing special operations and counterterrorism missions.

The military — and the CIA — would like to preserve the ability to launch drone strikes against militant targets inside Pakistan. “If we want an optimal solution,” the senior military official said, U.S. forces need “the physical ability to have the intelligence and strike assets that are there.”

“The enemy has a sanctuary in Pakistan,” the official said. “You have to address the challenges that are in Pakistan in order to be successful in Afghanistan.”