OTTAWA -- The United Nations has formally asked Canada to extend its mission in Mali in what appears to be a last-ditch effort to prevent a gap in the provision of military medical evacuations for wounded peacekeepers and UN staff.

The UN request is contained in a letter sent to the federal government at the end of February after months of quiet lobbying was met with steadfast resistance in Ottawa.

The move is somewhat unusual because such formal requests are often only made when the UN believes it has a good chance of success, which is anything but certain in this particular circumstance.

Yet the presence of a formal request also increases the pressure on Canada to respond positively after the government, including Prime Minister Justin Trudeau, repeatedly played down the gap.

The timing is also noteworthy given that the UN hosts a major peacekeeping summit in New York next week, the first such meeting since Canada hosted a similar event in Vancouver in November 2017.

Canada has eight helicopters and 250 military personnel in Mali, where they have been providing emergency medical evacuations and transporting troops and equipment across a large swath of the remote African country.

The contingent is scheduled to end operations at the end of July, at which point it will pack up and leave before a Romanian force arrives to take over.

However, the Romanians aren't scheduled to begin operations until Oct. 15, meaning there will be a roughly two-and-a-half-month gap needed to be filled.

In its Feb. 28 letter to the government, portions of which were read to The Canadian Press, the UN "kindly asks the government of Canada to consider a short extension of its contribution."

Specifically, it asks that Canada continue full operations until Sept. 15, and then more limited operations until Oct. 15 as the Canadians draw down and withdraw to make way for the Romanians.

The unsigned letter goes on to say that "such an approach will bridge the gap" until the Romanian contingent can begin operations.

The UN wanted a response by Friday. A UN official, speaking on condition of anonymity because of diplomatic sensitivities, said the federal government has since asked for a two-week extension.

Officials for Foreign Affairs Minister Chrystia Freeland, who is expected to attend next week's peacekeeping summit in New York, did not immediately respond to requests for comment.

The federal government previously argued that the UN can fill the gap between the departure of the Canadians contingent and arrival of the Romanians with civilian contractors, as it has done in the past.

The Liberals also suggested that Canada is actually supporting the UN by sticking to its schedule to end operations at the end of July, while downplaying the UN's concerns about the gap.

"We are confident, and we are hearing from the UN that there is no concern about that gap being a problem," Trudeau told reporters during a visit to the Mali mission in December.

The UN, which has faced shrinking budgets after the U.S. cut its funding to peacekeeping last year, argues that a short extension is more cost efficient given that Canada already has the people and equipment in Mali for the mission.

Contracting civilian helicopters in Mali costs about $1 million per month, the UN official said.

Civilian helicopters also aren't able to provide the same level of comprehensive medical treatment that the Canadians are set up to offer, the UN has said, and are more restricted in when and where they can operate.

NDP defence critic Randall Garrison, who called for the government to extend the mission after visiting Mali last month, said the letter "puts the lie" to the Liberals' arguments for not extending the mission.

He said not extending the mission "puts people's lives at risk in Mali and it puts our international reputation at risk now that we know the UN has formally asked for an extension."