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Canada is racing to wind down its Afghanistan evacuation effort ahead of America's 31 August deadline, as officials warn some may be left behind.

As of 24 August, Canada had airlifted more than 2,700 people out of Kabul, including Canadian citizens, Afghans and other foreign nationals.

But efforts have been stymied by chaos on the ground, Defence Minister Harjit Sajjan acknowledged on Wednesday.

"We are pushing our people and our aircrafts to their limits," he said.

Canada has vowed to take in 20,000 Afghan refugees, but has not specified a timeline.

Officials say the country's armed forces may have just days left to complete evacuation efforts before they are forced to withdraw. Canada's efforts rely on the American military's control of the Kabul airport, meaning it must wind down efforts before they leave.

"We would have liked to have stayed beyond the 31st deadline, but as you know this decision has been made by the Americans," Mr Sajjan said.

Foreign Affairs Minister Marc Garneau said on Wednesday it was possible that Canada would not complete its evacuation in time for the draw down.

He said the "commitment" will continue beyond the deadline, without offering specifics.

Reports out of Afghanistan have described a chaotic effort by the Canadian military to get people out of the country.

Criticism was amplified this week by cell phone footage posted to Facebook that appeared to show Canadian special forces ignoring the pleas of Afghans claiming to have Canadian exit documents.

Asked about the footage on Wednesday, Minister Sajjan called the scene "heart-wrenching", but defended the work of the military saying that they "have to establish security" at the airport.

"Security is what allows that work to continue to bring people in," he said.

Canada formally withdrew its own military presence in Afghanistan in 2014, but forces were sent back in recent days to assist rescue efforts.

The evacuation efforts come as most federal politicians, including Liberal leader Justin Trudeau, are on the campaign trail.

The prime minister called a snap election federal election earlier this month.