Canada's new Liberal government will act within weeks to fulfill a campaign promise to withdraw six fighter jets that have been attacking Islamic State positions in Iraq and Syria, a top official said on Monday.

The Liberals, who took power last month, say Canada can contribute more effectively to the US-led campaign against the militants by assigning more troops to train Kurdish fighters in northern Iraq.

"It's a matter of weeks, not months," Foreign Minister Stephane Dion told reporters when asked when the jets would be pulled out. Dion's comments were the most specific so far from a cabinet member about when the planes would return home.

"We're carrying out two per cent of the air strikes. We're going to do something more efficient for the coalition," he said.

Diplomatic sources say the United States, France and Britain have privately expressed their unease about Canada's pledge on the grounds it could undermine the effort to contain Islamic State.

The Liberals of Prime Minister Justin Trudeau took office after defeating the Conservatives, who took the decision to send the jets and trainers to the Middle East.

In Parliament on Monday, the Conservatives noted that when U.S. President Barack Obama on Sunday cited close allies who were taking part in the operations against Islamic State, he had not mentioned Canada.

"Why is the prime minister stepping back from the fight when our allies are stepping up?" asked interim Conservative leader Rona Ambrose.

Trudeau replied that he had "engaged with our allies on these issues, and they reassured me that we are continuing to be helpful".- Reuters