Stephen Harper heads into an emergency Group of Seven meeting spearheading the charge to kick Russia out of the G8 club of leading industrialized nations over its seizure of Crimea – a land grab that has triggered the greatest European security crisis in decades.

Canada is also pushing for all G7 countries to move in concert on further sanctions against Russia, seeking to avoid a situation where some go further than others on punitive actions and are then singled out for retaliation by Moscow. The alternative would mean some G7 economies feel the brunt of counter-sanctions by Russia while others bear no further cost of standing up to Mr. Putin.

On Sunday, the NATO alliance's top commander in Europe pushed already sky-high tensions even higher when he told a conference in Brussels that Russia had massed a large and battle-ready military force along its border with Ukraine.

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U.S. Air Force General Philip Breedlove's comments come before Mr. Harper arrives at the G7 conference Monday, on the sidelines of a long-scheduled nuclear security summit at The Hague. The Prime Minister is the only Group of Seven leader to have visited Ukraine since the crisis there began. He'll offer G7 counterparts a first-hand report of his conversations with Kiev decision makers when world leaders meet early Monday evening.

Mr. Harper journeyed to Kiev under heavy security Saturday for a tête-à-tête with Ukraine Prime Minister Arseniy Yatsenyuk. In a highly symbolic gesture, the Canadian Prime Minister stood shoulder to shoulder with Mr. Yatsenyuk and forcefully rebuked Russian President Vladimir Putin, whose military forces are massing on Ukraine's doorstep, and, Kiev fears, prepared to claim even more territory.

The Prime Minister accused Mr. Putin of returning international relations to the "law of the jungle" and warned that Moscow's seizure of Crimea would trigger an arms buildup around the world as countries grow more anxious about the security of their borders and more skeptical about Western efforts to halt nuclear proliferation. Canadian government officials say his Kiev statements form the core of the case Mr. Harper will make to G7 partners Monday, a forum where he is the most vocal advocate of expelling Russia from the Group of Eight.

Mr. Harper, who has previously cast Moscow as a pariah by calling the Group of Eight the "G7 plus one," left no doubt where he stands on the future of Russia's membership Saturday. "I don't think it takes much imagination to figure out what my view is but I will certainly listen to what our partners in the G7 have to say before we arrive at final decisions."

The Prime Minister laid out the arguments he will make for why half-measures against Mr. Putin will not be sufficient. Mr. Harper charged that Mr. Putin has violated the 1994 Budapest Memorandum where Ukraine relinquished its share of the Soviet arsenal of nuclear weapons "on the basis of an explicit Russian guarantee of its territorial integrity," he said. "By breaching that guarantee, President Putin has provided a rationale for those elsewhere who needed little more than that already furnished by pride or grievance to arm themselves to the teeth." Mr. Harper also announced he would restart free-trade talks with Ukraine, a show of support for the country's new leadership.

The Prime Minister said, if unchecked, Russia's takeover of Crimea "will be felt far beyond the borders of Ukraine or even the European continent itself."

Gen. Breedlove, NATO's top European commander, warned of the more immediate worries from the Russian military buildup at Ukraine's borders. "The force that is at the Ukrainian border now to the east is very, very sizable and very, very ready," Gen. Breedlove said at an event organized by the German Marshall Fund, a think tank. He suggested Russia now had enough troops at the ready to push through eastern and southern Ukraine, thereby connecting mainland Russia with the newly annexed Crimean Peninsula, as well as the breakaway pro-Russian region of Trans-Dniester, in the former Soviet republic of Moldova.

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"There is absolutely sufficient [Russian] force postured on the eastern border of Ukraine to run to Trans-Dniester if the decision was made to do that, and that is very worrisome," Gen. Breedlove said, adding that NATO forces should be redeployed to protect the alliance members Estonia, Latvia and Lithuania, which share borders with Russia.

Russia's deputy defence minister, Anatoly Antonov, expressed bewilderment at the claims, saying his country was not undertaking in any undeclared military activity. He said Russia was observing internationally agreed limits regarding the number of troops it could station along the border with Ukraine, and that Russia's military positions had recently been visited by observers from the Organization for Security and Co-operation in Europe.

"Russian armed forces are not involved in any manner of unannounced military manoeuvres that would endanger the security of neighbouring states," Mr. Antonov said. "We have nothing to hide."

But Ukraine's acting foreign minister, Andriy Deshchytsya, said he believed war with Russia was a "growing" possibility. "This situation is becoming even more explosive than it used to be a week ago," he told ABC's Meet the Press program.

Gen. Breedlove said that Russia would currently have a tactical advantage in any conflict because its troops were already engaged in large-scale exercises in the border regions. While Ukraine is not a North Atlantic Treaty Organization member, Gen. Breedlove called for NATO forces to be redeployed to protect the alliance members Estonia, Latvia and Lithuania, which do share borders with Russia.

"I think we need to think about our allies, the positioning of our forces in the alliance and the readiness of those forces … such that we can be there to defend against it if required, especially in the Baltics and other places," he said.

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Gen Breedlove expressed specific worries that Trans-Dniester could be the next region to see Russian military action. The tiny, landlocked strip has had de facto independence from Moldova since a 1992 war, and already hosts a small contingent of Russian soldiers.

Mr. Yatsenyuk offered a brief moment of levity Saturday when he joked that Ukraine would be happy to replace Russia in the Group of Eight. "If G8 has an empty seat, we are ready to take it," he said.

The Ukrainian prime minister also called on the Group of Seven to move beyond economic cooperation and take on a new role in global security. "My message to G7 members: It's time to build up a new security system, as quickly as possible, and to [answer] those who violate this system with a strong response," Mr. Yatsenyuk said.

Mr. Harper made sure to mention the huge Ukrainian diaspora in Canada at the start of his public statement alongside Mr. Yatsenyuk. The Conservative Leader is very mindful of the fact that his unqualified support for Kiev plays well at home.

"Prime Minister, I bring you greetings from all Canadians including, but certainly not limited to, more than one million Canadians of Ukrainian heritage," Mr. Harper said. "The third largest Ukrainian population in the entire world."

He told Ukrainians that Canada holds their cause dear.

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Mr. Harper ended his statement by saying "Glory to the Ukraine," a rallying cry used by protestors in Kiev as they demonstrated against the government of now ousted president Viktor Yanukovych.