Former U.S. secretary of state Hillary Clinton on Tuesday accused Russian President Vladimir Putin of attempting to "rewrite the boundaries" of post-World War II Europe.

It came as Putin signed a treaty claiming the Black Sea region of Crimea as Russian territory, as Ukraine warned the showdown had entered a "military stage" after soldiers were killed on both sides.

President Vladimir Putin on Tuesday signed a treaty claiming the Black Sea region of Crimea as Russian territory as Ukraine warned the showdown had entered a "military stage" after soldiers were killed on both sides.

The treaty signing was conducted at lightning speed in the Kremlin in a defiant expansion of Russia's post-Soviet borders that has plunged relations with the West to a new post-Cold War low.

Ukraine's Western-backed prime minister said on Tuesday that his country's conflict with Russia was entering a "military stage" following claims by Kiev that one of its officers was shot and killed in Crimea.

"The conflict is shifting from a political to a military stage," Prime Minister Arseniy Yatsenyuk told an emergency government meeting. "Russian soldiers have started shooting at Ukrainian military servicemen, and that is a war crime."

Its borders are patrolled by Russian troops and it adopted the ruble on its first day of "independence" -- welcome to the "Republic of Crimea".

Now claimed by Moscow as part of its territory, Crimea is still officially deemed part of Ukraine by the rest of the international community and its separatist leaders have already been hit with a series of EU and U.S. sanctions.

U.S. President Barack Obama on Tuesday called for a G7 summit next week to discuss the escalating showdown with Russia over Crimea.

The White House said Obama asked fellow leaders of the grouping -- minus G8 member Russia -- to join him in the Hague, where he will be attending a nuclear security summit, to discuss the crisis and how to support Ukraine.

Britain warned Tuesday that the West and Russia faced a changed relationship in the coming years, as London suspended all bilateral military cooperation and halted arms exports to Russia.

Foreign Secretary William Hague said President Vladimir Putin had chosen the "route of isolation" by signing a treaty annexing Crimea just two days after a hastily arranged referendum on the breakaway peninsula.

Ukraine's new Western-backed prime minister said Tuesday that the ex-Soviet country had no plans to join NATO following last month's fall of a pro-Kremlin regime.

Prime Minister Arseniy Yatsenyuk delivered a special address to the nation designed to ease tensions between Ukrainian nationalists who spearheaded three months of protests against the pro-Moscow authorities and Russian speakers who view the new pro-European government in Kiev with mistrust.

President Vladimir Putin on Tuesday signed a treaty making Crimea part of Russia, in a historic redrawing of Russia's borders after he declared that the Black Sea region has always been "in the hearts" of his countrymen.

In a fast-moving sequence of events following Crimea's controversial secession referendum on Sunday, the Kremlin said Crimea was now considered part of Russia and no longer Ukrainian territory, shrugging off strong international objections.

Japan on Tuesday said it would slap sanctions on Russia over its "deplorable" move recognizing a Crimean vote to break away from Ukraine.

Russian President Vladimir Putin on Monday signed a decree recognizing Crimea as an independent state following a weekend referendum to secede from Ukraine and join Russia in a move that has fanned the worst East-West crisis since the Cold War.

A Russian diplomat who was allegedly stabbed by a Canadian army reservist last week has left Canada, police confirmed Monday after the soldier was granted bail.

Ottawa police Constable Marc Soucy told Agence France Presse the Russian embassy worker identified in court documents as Andrey Gorobets "has left the country."