NATO's top military commander is raising the prospect of stepping up Western military exercises as a response to large-scale Russian troop training operations near Ukraine's border.

General Philip Breedlove, NATO's Supreme Allied Commander, Europe, broached the idea during an address to an Ottawa audience Monday where he discussed the alliance's need to reinvent itself in light of Russia's move on Ukraine.

Stephen Harper, who met with Gen. Breedlove earlier in the day, called Russia's actions in Eastern Europe a "slow-motion invasion" of Ukraine.

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His comments came the day pro-Russian rebels shot down a Ukrainian helicopter in fierce fighting near the eastern town of Slavyansk, and Kiev drafted police special forces to the southwestern port city of Odessa to halt a feared westward spread of rebellion.

Gen. Breedlove posed a question during his address on whether the North Atlantic Treaty Organization needs to ramp up its soldiering exercises to match the scale of Moscow's. The Western alliance has estimated that Russia's recent exercises in its western military district near Ukraine increased Moscow's troop presence in the region to 40,000.

"Do our exercises and readiness measures need to go beyond current Connected Forces Initiatives in order to match the capacity of, for example, the Russian snap exercises we have witnessed over the past several months?" Gen. Breedlove said.

Moscow's annexation of the Crimean Peninsula and its support for Russian separatists in eastern Ukraine have filled NATO with renewed purpose. The Western alliance has greatly stepped up air patrols and troop exercises in member countries that are neighbours of Russia.

The 65-year-old military alliance, formed in part to counter Soviet Union expansionism, now needs to consider whether it should beef up its readiness and capacity to respond to threats, the NATO commander said.

"Russia's aggression in Ukraine has caused a paradigm shift," Gen. Breedlove said.

"And in response, NATO must and will adapt to remain credible and relevant."

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The military commander said NATO requires a major review of its capacity to defend its members and respond to crises.

"It is time to ask ourselves some hard questions," he said.

"Are we structured correctly to provide a rapid and credible response? Is the alliance agile and flexible enough to react appropriately?" the commander said. "And even more tough: Are our forces positioned correctly to respond?"

Gen. Breedlove said the Ukraine crisis has affirmed the strength of the "trans-Atlantic bond" between NATO members but also highlighted weaknesses in the alliance's structure, its mechanisms to respond to trouble and the tools at its disposal.

"As we prepare for the NATO summit in September … we need to clearly articulate our role in deterrence and then, if required, defence."

Canada has contributed a warship, six fighter jets and more than 300 troops to European exercises designed to reassure NATO members neighbouring Russia that they will be protected from Moscow's territorial ambitions.

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Parliament has yet to debate this military commitment and the Prime Minister's Office says what Canada has contributed to date does not require such a Commons discussion.

"As a member country, Canada has responded to NATO's call for support for this important reassurance mission, which is very different from, say, committing troops to a combat mission," said Jason MacDonald, director of communications for the Prime Minister's Office.

"The opposition is always free to request a debate or use an opposition day motion to debate the issue."

Mr. MacDonald said that in the past year, Ukraine has been the topic of more special debates in the Commons than any other issue.

Ukraine said its new Odessa force, based on "civil activists," would replace local police who had failed to tackle rebel actions at the weekend. Its dispatch was a clear signal from Kiev that, while tackling rebellion in the east, it would vigorously resist any sign of a slide to a broader civil war.

In the east, fighting intensified around the pro-Russian stronghold of Slavyansk, a city of 118,000, where rebel fighters ambushed Ukrainian forces early in the day.

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Russia's foreign ministry called on Kiev to "stop the bloodshed, withdraw forces and finally sit down at the negotiating table."

With files from Reuters