German Foreign Minister Frank-Walter Steinmeier said he had told his Russian counterpart Wednesday that approval is needed immediately for a mandate sending foreign observers to Ukraine, as Berlin halted a major arms deal with Moscow.

"I said again this morning in the conversation that the mandate really needs to go through in the next 24 hours," Steinmeier told reporters after telephone talks with Sergei Lavrov and the chairman of the Organization for Security and Cooperation in Europe (OSCE), Didier Burkhalter.

U.S. Vice President Joe Biden on Wednesday said Washington could send forces to the Baltic states to reassure ex-Soviet NATO allies rattled by Russia's takeover of Crimea.

"We are exploring a number of additional steps to increase the pace and scope of our military cooperation, including rotating U.S. forces in the Baltic region to conduct ground and naval exercises and training missions," Biden said during a visit to Lithuania, after a first stop in Poland.

British Prime Minister David Cameron said Wednesday that G7 nations meeting next week must discuss the permanent expulsion of Russia from the wider G8 if it takes further action to destabilize Ukraine.

U.S. President Barack Obama has called for a G7 summit -- minus current G8 chairman Russia -- on the sidelines of a nuclear security summit in The Hague on Monday and Tuesday to discuss the escalating showdown over Russia's annexation of Crimea.

Australia on Wednesday said it will impose targeted financial sanctions and travel bans against officials involved in the annexing of Crimea, rallying behind allies the United States and the European Union.

While none of the 12 Russian and Ukrainians in the firing line was named, Foreign Minister Julie Bishop said they were "individuals who have been instrumental in the Russian threat to the sovereignty and territorial integrity of Ukraine".

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.