Canada has told US President George W Bush that it will withdraw its troops from Afghanistan next year unless Nato deploys more soldiers there.
Nato and Afghanistan want Canadian troops to stay on
Prime Minister Stephen Harper is demanding that a further 1,000 Nato troops be sent to Kandahar province where Canada's 2,500 troops are based.
Canada's current mission in Afghanistan is set to expire in February 2009.
Seventy-eight Canadian soldiers and one diplomat have been killed in Afghanistan since 2002.
An independent report into Canada's mission issued last week recommended that Canadian troops remain in Afghanistan indefinitely.
However, it also said that Canada should pull out if it did not get the support it needs.
A spokeswoman for Mr Harper said the prime minister had told Mr Bush that "unless Canada was able to meet the conditions specified by the [report] of additional combat troops and equipment from Nato allies, Canada's mission in Afghanistan will not be extended".
White House press secretary Tony Fratto said that in reply, the president had "noted the deployment of 3,200 additional US marines to Afghanistan".
We hope Canada will find a way to extend the mission
It is not clear if the additional US troops meet Canada's requirements for more troops in Kandahar.
Nato spokesman James Appathurai said the alliance would discuss Canada's demand at a meeting next month, and would try to find the additional troops.
"Nato thinks Canada is doing a very important and valuable job in Kandahar. We hope Canada will find a way to extend the mission," he said.
There was a long-standing request to Nato countries to provide additional resources, he added.
Canadian troops are engaged in counter-insurgency operations against the Taleban, alongside mainly British, US and Dutch troops.
Mr Harper has always supported extending Canada's military mission.
To do so beyond the 2009 deadline would require the approval of parliament.
But the Liberals and other opposition parties all want Canada's direct combat role to come to an end.