Nato defence ministers will meet this week to discuss its future military presence in Afghanistan as Barack Obama weighs whether to order the Pentagon to begin preparations for a total withdrawal of all US forces at the end of this year.

The crunch meeting in Brussels on Wednesday comes amid growing acceptance in Washington that Afghan president Hamid Karzai is unlikely to sign a bilateral security agreement (BSA) finalised last November for a residual US security force to remain after the bulk of its troops pull out in 2014.

Afghanistan has elections scheduled for April to choose a successor to Karzai, who as incumbent cannot stand again. But a complicated two-round voting system means there is little expectation a new leader will be in office before the end of the summer.

Instead, the US is increasingly focused on making a deal with whoever succeeds him. All of the top contenders have backed the BSA, but by the time the winner comes into office, Nato may only have a few months to plan its 2015 deployment.

Although the last troops could still be withdrawn relatively quickly if needed, the political impasse threatens to frustrate the complex process of scaling down the Nato presence – particularly for allies that depend on the US for logistical support.

As a result, the White House has been considering whether to order to its military commanders to include the so-called “zero option” in its planning scenarios, according to a senior official speaking on background.

Though symbolic to the extent that Pentagon planners have long looked at a range of possible options for troop numbers, the formal order to plan for a full withdrawal would mark a significant escalation in the US war of words with Karzai.

Administration officials say that as of the middle of last week, president Obama had yet to make a final determination on the order but is keen to demonstrate that he is not bluffing when he says the US would pull out entirely if the BSA is not signed.

Military officials, who have worked through months of uncertainty over the future US role in Afghanistan, agree that the forthcoming Nato meeting represents a key moment in the tussle with Karzai.

“Without question, the issues surrounding the future of the Afghan mission will certainly be discussed in Brussels, to include the challenges of retrograde and force posture faced by all Allies,” said John Kirby, the Pentagon press secretary.

“We long said that we would like a BSA as soon as possible. Absent that, we would have to start planning to withdraw all our forces by the end of the year,” Kirby added. “I’m not going to put any timelines on that. We’d like a BSA as soon as possible.”

The prospect of the US retreating entirely from Afghanistan without being able to guarantee a continued counter-terrorism role would carry significant risks for the White House, which is conscious that the US has spent 13 years and 2,000 soldiers have died trying to stabilise the country.

However, it believes that US public opinion is supportive of the withdrawal process and would be sympathetic to arguments that a continued presence is not wanted by the Afghans.

Security setbacks in Iraq following a similar failure to agree a US presence there have led to relatively little domestic criticism, despite the recent symbolic loss to insurgents of Fallujah.

The White House acknowledges it would be more difficult to carry out successful counter-terrorism operations against international militants along the lawless border with Pakistan if they do not have some troop presence on the ground, but privately argues such a role is not impossible – even without drone bases in the country.

In Afghanistan, Karzai’s about-turn on the BSA late last year took most of the country, including portions of his own cabinet, by surprise.

The president’s national security adviser Rangin Dadfar Spanta had already warned parliament that if the deal was not signed, the country would be left to the wolves – a warning about Afghanistan’s fate in a region of wealthier and more populous neighbours keen to to have a say on policy in Kabul.

Most senior Afghans have not forgotten the fate of the last government backed by a global superpower. Despite forecasts of rapid collapse after Soviet troops left in 1989, President Mohammad Najibullah held off mujahidden fighters until 1992. But when the disintegration of the Soviet Union ended funding for the military, the government fell within weeks.

So worries in Kabul are not so much about the troops that the US may or may not leave behind. Without their training and support the Afghan army will probably be less professional, and certainly struggle in critical areas from intelligence gathering to air power, but would probably still be able to hold off the Taliban.

The concern is about the money that the soldiers, and the drone bases they would support, bring with them.

Few in Kabul think the west – and particularly Washington – would be willing to provide a promised $8bn a year in military and development spending if they have been effectively booted out by the Afghan government. “Without the troops, there is no money,” said one diplomat.

The leading presidential candidates have all backed the strategic pact, with the 2009 runner-up Abdullah Abdullah slamming Karzai for feeding isolationist sentiment in the US.

His main rivals have openly said they would sign the deal. Zalmai Rassoul, a former foreign minister believed to have Hamid Karzai’s blessing, has described it as “good for Afghanistan” and promised a signature if he wins.

Former finance minister and World Bank technocrat Ashraf Ghani helped hammer out the pact and described it as “the source of Afghanistan’s national sovereignty”, underlining that it gives only the Afghan state the right to use force.

But after years of aborted ultimatums on issues from night raids on homes to control of prisoners – all areas where the US has ultimately bowed to Karzai’s demands – American deadlines have limited credibility in Kabul.

There are fears among those familiar with the climate in Washington that Afghan officials are underestimating the extent of American disengagement from the war.

“We don’t believe there is a zero option,” Karzai’s spokesman told journalists last year after the president’s surprise retreat from the long-term strategic partnership.

Although that infuriated many in Washington, the decision of top officials there to set Karzai a deadline for a decision on the BSA, only to back away from it when it passed unheeded, strengthened the belief of the Afghan leaders’ inner circle that the US was ultimately committed to keeping troops in Afghanistan regardless of the political and financial cost.