US President Joe Biden's decision to withdraw troops from Afghanistan by September 11 has left Washington and its allies scrambling to work out how to protect the diplomats and officials staying behind.
As he announced the end of America's "forever war" in April, Biden pledged his administration would "determine what a continued US diplomatic presence in Afghanistan will look like" and how to keep it safe.
Chief among the challenges now confronting Western officials is that of maintaining security at Kabul airport -- the vital link between the foreign embassies clustered together in the city's fortified "green zone" and the outside world.
As fears mount over a Taliban comeback once US and allied NATO troops leave after almost two decades on the ground, there are major unanswered questions about how to maintain a facility vital for Afghans as well as foreigners.
"That is one of the keys to maintaining a diplomatic presence," Chairman of the Joint Chiefs of Staff Mark Milley said Tuesday after a discussion with other NATO military heads in Brussels.
"So we are working out the details of how to secure the airport, how to support the Afghan military securing the airport, and what countries are willing to contribute to do that.
"And there was much discussion about that."
Milley said leaders from NATO's 30 nations could take more decisions on future plans for Afghanistan when they meet on June 14 at a summit in Brussels.
- 'Huge concern' -
But for now the strategy remains up in the air.
A NATO official told AFP only that the alliance "remains committed to its enduring partnership with Afghanistan and to supporting the sustainment of the Afghan security forces."
"We are now looking into the details on how we can continue to provide support," the official said.
The Pentagon estimates that it takes around 1,000 to 1,500 soldiers to assure security at the airport, currently handled by NATO member Turkey, with support from Afghan troops.
As foreign troops eye the exit door, Afghanistan's special forces are seen as the only local contingent capable of managing the task.
But if the Taliban continues its offensive, these troops would most likely be required elsewhere to help stave off a collapse of the government.
US and European officials say several other options are being mooted to try to protect the airport.
These include getting civilian contractors to handle security, individual countries agreeing to stay on independent of NATO, or turning to the United Nations.
The US delegation said several countries expressed a willingness at the meeting of NATO defence chiefs to be involved in securing the airport to try to ensure a continuing presence in Kabul.
A Western diplomatic source said Turkey is considering maintaining its presence but nothing has been promised so far.
Allies fear though that the presence of any foreign forces -- private or national -- could be seen as a major affront by the Taliban and could definitively derail a stuttering peace process.
Washington insists there will not be any American military presence or US contractors after September 11.
NATO is also ruling out keeping forces in the country under the alliance's umbrella.
The uncertainty surrounding future security arrangement has left European diplomats fretting over keeping personnel in the country to supervise projects such as like keeping economic assistance flowing and funding the Afghan armed forces.
They say that having a functioning and safe airport is a minimum pre-condition for remaining.
EU countries want to make sure "that through all of this the gains that have been made...around human rights, around rules of law, around democratization over the last 20 years are not lost in the process," said one European diplomat.
"There's a huge concern at the moment just about basic issues like physical presence on the ground. That's the level that we're at."