Ukrainian Prime Minister Arseniy Yatsenyuk is warning Russia that military intervention would mean war between the two neighbors, after the Russian parliament authorized the use of Russian troops in the southern Ukrainian region of Crimea.
Acting Ukrainian President Oleksandr Turchynov said Saturday he has put the country's armed forces on combat alert following Saturday's vote in Russia.
The approval of Russian President Vladimir Putin's request for military authorization made official what Ukraine's newly-established interim government has described as an ongoing deployment of Russian troops in Crimea, a Black Sea peninsula.
The majority-Russian region, which became part of independent Ukraine when the Soviet Union collapsed in 1991, has become the focus of turmoil that began months ago when Ukraine's now-ousted president Viktor Yanukovych rejected a deal with the European Union in favor of closer ties with Russia.
The United Nations Security Council scheduled an emergency meeting on Ukraine Saturday, while European Union foreign ministers will hold emergency talks in Brussels on Monday.
The situation has escalated into the biggest confrontation between Russia and the West since the Cold War.
On Saturday, Ukraine's U.N. mission asked members of the Security Council to "do everything possible" to stop what it described as Russian "aggression" against Ukraine, calling Russia's actions a "severe violation of international law."
U.N. Secretary-General Ban Ki-moon's spokesman said Saturday the U.N. chief is "gravely concerned about the deterioration of the situation" in Crimea and "reiterates his call for the full respect" for Ukraine's independence, sovereignty and territorial integrity. The spokesman said Mr. Ban would be speaking with President Putin soon about the situation.
The United States has also urged Russia to respect Ukraine's territorial integrity. Russia's upper house of parliament recommended Saturday that Mr. Putin recall the Russian ambassador from Washington, a day after U.S. President Barack Obama warned of "costs" if Russia intervenes militarily in Ukraine. President Obama's national security team met at the White House Saturday to discuss possible policy options.
Mr. Putin's spokesman, Dmitry Peskov, meanwhile, said the Russian president has not yet decided whether to use troops in Ukraine and will make the decision based on how the situation develops. He said Mr. Putin has also yet to decide whether to recall Moscow's ambassador to the United States.
As the tensions surrounding Crimea escalated, pro-Russian protesters clashed violently with supporters of the new Ukrainian government in Ukraine's eastern city of Kharkiv Saturday. Pro-Russian demonstrations also erupted in other eastern cities.
Earlier, the newly appointed pro-Russian prime minister of Crimea, Sergei Aksyonov, claimed control of the region's military and other security forces. He appealed to Mr. Putin for help in restoring "peace and calm."
Russia has said its troop movements in Crimea, where it leases a naval base in Sevastopol, conform to agreements with Ukraine. But Ukraine's acting defense minister said 6,000 additional Russian troops have been deployed on Ukrainian soil.
Ukraine also has refused to recognize the Crimean prime minister, with President Turchynov issuing a statement declaring Mr. Aksyonov's appointment a violation of Ukraine's constitution.
Mr. Aksyonov was appointed by the Crimean parliament as tensions soared over Crimea's resistance to the new authorities in Kyiv.
VOA correspondent Elizabeth Arrott says unidentified soldiers and military vehicles have appeared in the Crimean region, well beyond their local base. She said at least a dozen were stationed outside parliament in the Crimean capital of Simferopol on Saturday. Arrott also said she saw gunmen in camouflage at the Simferopol airport Friday.
There also are reports of Russian troops surrounding the state-run television station in Simferopol.
The head of Russia's upper house of parliament said Saturday she could not rule out the possibility that a limited contingent of troops could be sent to Crimea to ensure the security of the Black Sea fleet in Sevastopol and Russian citizens living in the region.
Crimea, placed under Ukrainian control in 1954 by then-Soviet leader Nikita Khrushchev, has a tiny border with Russia on its far eastern point. Most of the people living in Crimea are ethnic Russians, but the region also is home to ethnic Muslim Tartars who generally show disdain toward Russia.
Ukrainian President Turchynov on Friday called Russian actions in Crimea "naked aggression." He likened the actions to events that led up to Russia's 2008 invasion of Abkhazia - a pro-Russian region of Georgia.
In another development Saturday, Russia's Energy Ministry threatened not to continue Ukraine's gas price discount because of Ukraine's unpaid balance. Gazprom said Saturday that Ukraine's outstanding gas debt for 2013 and this year is $1.55 billion.