U.S. President Barack Obama announces Wednesday that Gen. David Petraeus will replace Gen. Stanley McChrystal as top commander in Afghanistan. (Larry Downing/Reuters)

The U.S. ambassador to Afghanistan promised Thursday to work on a unified mission with Gen. David Petraeus after the ouster of Gen. Stanley McChrystal as the top commander in the war.

President Barack Obama replaced McChrystal as head of U.S. and NATO forces in Afghanistan on Wednesday after a magazine article in which McChrystal and his aides criticized the U.S. administration.

"The United States cannot allow diversions to prevent us from carrying out our mission with unity of purpose," Ambassador Karl Eikenberry said in a speech to Afghan journalists. "Our president felt that a change was needed to maintain that unity of purpose and so he made that change. He told us that it is time for us to come together and that's what we are going to do."

Eikenberry, who had differed with McChrystal on Afghan strategy, said he was confident Petraeus, who has been involved in creating and implementing the Afghan strategy, would be able to take up the new post without losing momentum.

But some of the mission's international partners were already expressing doubts. The head of Poland's National Security Office said NATO should change its strategy in Afghanistan or face a fiasco, describing the situation there as growing "systematically worse."

Canada's top soldier, however, backed the change, vowing Canadian operations in Afghanistan will continue "without issue," despite the recent changes in the U.S. military leadership.

Gen. Walt Natynczyk, Canada's chief of defence staff, said Petraeus is "a highly experienced commander," who will lead NATO forces with "great skill, courage and determination."

"I have served alongside Gen. Petraeus on operations and he has been an instrumental part of the leadership team overseeing the mission in Afghanistan," Natynczyk said in a statement.

A U.S. Senate panel is moving quickly to hold a confirmation hearing for Petraeus. The armed services committee will hold a hearing for the four-star general next Tuesday.

Petraeus currently heads the U.S. Central Command, which oversees military operations in Iraq, Afghanistan and elsewhere. His replacement has not been named.

U.S. General Stanley McChrystal, seen in this June 2010 photo, resigned over comments he made in a magazine article. (Massoud Hossaini/Retuters)

McChrystal apologized Tuesday for comments made in a Rolling Stone article, which described McChrystal as a lone wolf, on the outs with many important figures in the Obama administration. He was later recalled to Washington, where he offered his resignation.

"The conduct represented in the recently published article does not meet the standard that should be set by a commanding general," Obama said Wednesday.

"It undermines the civilian control of the military that is at the core of our democratic system."

Afghan leaders support swap

The Afghan president's office — which had strongly backed McChrystal — said it was relieved at the choice of Petraeus because he knew the war and would not change the strategy.

Afghan Defence Ministry spokesman Gen. Mohammad Zahir Azimi also voiced his support for McChrystal, and for Petraeus.

Until Petraeus is confirmed by the U.S. Senate, British Lt.-Gen. Nick Parker, the deputy commander of the NATO-led forces, is assuming command of the troops, according to British Prime Minister David Cameron. It is unclear how long the transition will take or what it will mean for operations now in planning stages.

Operations appeared undisturbed as NATO reported a number of strikes in and around the southern city of Kandahar in the past two days.

With files from CBC News