The Trudeau government has decided it will send troops to join a NATO high-readiness brigade preparing to deploy in Eastern Europe.

Defence Minister Harjit Sajjan says Canada will take a leadership role and establish one of the battle group formations requested by the alliance.

The impending dispatch of forces builds on the former Conservative government's placement of troops in Eastern Europe for training exercises and to reassure governments in the region that western nations were serious about holding Russian expansionism in check.

With Crimea still under Moscow's rule and war raging in eastern Ukraine, Sajjan acknowledged Friday the fielding of the new NATO brigade is a serious step, but one that Canada is prepared to wholeheartedly support.

"As part of NATO we were giving assurance to member states there, but now this has evolved to deterrence," the minister said.

The shift in language is important.

The roughly 200 Canadian troops that have been training with allies in Poland will assume a different posture and signal their willingness to stand up to any potential interference in their host nation. A battle group also has the potential to bring with it heavy equipment, including tanks, artillery and surveillance systems.

But Sajjan insisted the build up does not exclude open dialogue with Russia, as the Liberals promised in the last election.

Hundreds of troops

The official announcement comes just one day after U.S. President Barack Obama challenged Canada to do more to support the military alliance.

Sajjan declined to release further details about equipment or force size, saying there will be a formal announcement at the upcoming NATO summit in Warsaw.

Multiple sources told CBC News the deployment would involve hundreds of troops and that it will "happen in short order." They will fill out one of four battalions being assembled to act as a deterrent against Russian expansionism and to reassure jittery allies, particularly in the Baltic region.

The U.S., Britain and Germany have already agreed to fill out the other three battalions, which are expected to be sprinkled among the former East Bloc countries.

During his speech to Parliament on Wednesday, the U.S. president's most pointed remarks involved Canada's commitment to NATO.

"As your NATO ally and your friend, let me say, we'll be more secure when every NATO member, including Canada, contributes its full share to our common security," Obama said. "Because the Canadian Armed Forces are really good. And if I can borrow a phrase, the world needs more Canada. NATO needs more Canada. We need you."

Obama: 'The world needs more Canada' 1:21

The deployment would be a "core" contribution, meaning that Canadians would fill the slot permanently until NATO dissolves that force, said one source.

It would require the army to rotate one of its infantry battalions and a headquarters — perhaps as many as 500 troops — into the position once every six months.

Sajjan confirmed Canada will establish a persistent presence in the region, creating a command and structural backbone for the battalion, but he refused to discuss numbers and specifics.

"Like all deployments, we will conduct (this) deployment on a rotational basis," he said."When it comes to exact numbers and time frames, the prime minister will announce that."

Latvia, one of the three Baltic states, is expected to be the location for the Canadian contingent.

A spokesman for the Russian embassy in Ottawa criticized the decision, suggesting there were more productive things Canada could be doing.

"We believe that NATO build-up on Russia's doorstep, which is reminiscent of Cold War saber-rattling, is a complete waste of money and resources, diverting them from the real existential threat of international terrorism," Kirill Kalinin told CBC News in an email.

Conservative defence critic James Bezan withheld judgment on the decision, saying the size and scope of the force will determine whether the Liberals are providing NATO with a credible deterrence.

"However, following today's announcement many questions remain unanswered; how many troops will be deployed, where will they be stationed, what will they be doing and for how long? These details are essential for our troops and our allies," he said.

NATO builds a bigger force

Russia's annexation of Crimea and support of rebels in eastern Ukraine prompted NATO to increase the size of its existing rapid reaction force to 40,000 soldiers.

In a crisis, that force is expected to assemble and respond within a week to 10 days.

But at the last summit of alliance leaders in Wales, the 28 member countries agreed to create high-readiness brigade of 4,000 troops which could be in place within 48 hours and would be available to deploy into eastern Europe within 48 hours.

The new NATO force is a bone of contention for Russia and some, including Germany's foreign minister, have suggested it could be seen as a provocation.

When the Cold War ended, Moscow signed a treaty with its former adversary and in it the military alliance explicitly agreed not to station troops along the Russian border in former satellite states.

NATO officials now argue Russia effectively threw out the treaty with the annexation of Crimea and that it has a duty to defend new members, including the Baltic states, Poland and Romania.