The rapidly unfolding crisis in Ukraine is set to be the focus of talks between U.S. President Barack Obama and European Union leaders Wednesday in Brussels, Belgium.
Russia's formal annexation last week of Ukraine's Crimea region has sparked the biggest East-West confrontation since the end of the Cold War.
Meanwhile, Moscow's massing of troops near Ukraine's eastern borders has worried the interim government in Kiev -- as well as causing ripples of concern in other former Soviet republics that now belong to the EU and NATO.
Wednesday's EU-U.S. summit in Brussels comes on the heels of talks on the sidelines of a nuclear security summit in The Hague, the Netherlands.
Obama will also meet with NATO Secretary General Anders Fogh Rasmussen while in Brussels.
Speaking at The Hague on Tuesday, Obama said Russia had a way out of tensions over the crisis: Negotiate with Kiev and be prepared to "act responsibly" and respond to international norms, such as respecting Ukraine's territorial integrity.
If Russia doesn't act responsibly, "there will be additional costs" that could hurt the global economy but will affect Russia most of all, Obama said.
The U.S. president said Russia's annexation of Crimea "is not a done deal" because it's not internationally recognized.
But he acknowledged that the Russian military controls Crimea, and said the world can make sure, through diplomacy and sanctions, that Russia pays a price.
Ukraine: We need support
Russia insists its actions are legitimate and denies having used its armed forces in Crimea, saying the troops that took control of key installations were local "self-defense" forces.
Russia also insists the government in Kiev is illegitimate because ousted President Viktor Yanukovych, a close ally of Moscow's, was forced out in an armed coup. Yanukovych's ouster followed months of street protests sparked by his decision to ditch an EU trade deal in favor of closer ties to Russia.
In an interview Tuesday with PBS, acting Prime Minister Arseniy Yatsenyuk said Ukraine is struggling to maintain a fighting capability after it was "deliberately dismantled" under Yanukovych.
"What we need is support from the international community. We need technology and military support to overhaul the Ukrainian military and modernize -- to be ready not just to fight, but to be ready to win," Yatsenyuk said.
With an estimated 30,000 Russian troops now positioned near Ukraine's eastern border, Yatsenyuk repeated his pledge to defend Ukrainian territory.
His government ceded Crimea without a shot to demonstrate to the world that Russia was the aggressor, he said -- but if Moscow moves against another portion of Ukraine, the duty of all Ukrainians is "to protect our country," he said. "We will fight."
Moscow tightens grip
The United States and EU are seeking to exert pressure on Russia through a combination of sanctions and diplomatic isolation.
But Moscow has so far doggedly pursued its own course, even as Western leaders have denounced its actions as violations of Ukraine's sovereignty and a breach of international law.
Amid heightened tensions within Ukraine, the Russian Foreign Ministry on Wednesday accused the Ukrainian border service of refusing to let air crew off Aeroflot jets for rest periods after landing in Ukraine. Aeroflot is the Russian national carrier.
This "breaks the international acts in compliance with flight safety requirements," the ministry said in an online statement.
Meanwhile, Russia is tightening its grip on Crimea.
Crimea belonged to Russia until 1954 when it was given to Ukraine, which was then part of the Soviet Union. The region has a majority ethnic Russian population and other historic ties to Russia.
A large majority of its population voted in favor of joining Russia in a controversial referendum 10 days ago. Russian lawmakers in turn swiftly voted to absorb the Black Sea peninsula, where Russia has a major naval base, into the Russian Federation, and President Vladimir Putin signed the treaty into law.
In another step to cement the process, the vice-speaker of the Crimean parliament, Sergei Tsekov, was made a senator in Russia's upper house Wednesday, Russia's state-run ITAR-Tass news agency reported.
At the same time, Kiev has ordered the withdrawal of Ukrainian armed forces from Crimea, citing Russian threats to the lives of military staff and their families -- effectively yielding the region to Moscow's forces. They stormed one of Kiev's last bases there Monday.
Aleksey Chaly, often referred to as Sevastopol's new de facto mayor, announced Tuesday the dismissal of all "self-defense" teams, saying the "enemy" was now gone, as no forces loyal to Kiev remain in the city.
"I would like to draw the attention of some commanders of the self-defense units to the fact that the revolution is over," he said in a video published on YouTube. "This week, federal agencies are being established, and we're beginning to live by the laws of the Russian Federation."
The G7 group of leading industrialized countries has condemned both the Crimean vote to secede and Russia's annexation of Crimea. As a result, Russia has now been excluded from what was the G8.
-- CNN's Marie-Louise Gumuchian, Victoria Butenko, Radina Gigova and Boriana Milanova contributed to this report.
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