Ukraine put its armed forces on full combat alert on Saturday and warned Russia that any military intervention in the country would lead to war.
After a more than three-hour meeting with security and defense chiefs, Acting President Oleksander Turchinov said there was no justification for what he called Russian aggression against his country.
Standing beside Turchinov, Prime Minister Arseny Yatseniuk said he had urged Russia to return its troops to base in the Crimea region during a phone call with Prime Minister Dmitry Medvedev and called for talks.
"Military intervention would be the beginning of war and the end of any relations between Ukraine and Russia," Yatseniuk told reporters.
Interfax news agency quoted a Ukrainian military source as saying, two Russian anti-submarine warships have appeared off the coast of Ukraine's Crimea region.
The source said the two vessels, part of Russia's Baltic Fleet, had been sighted in a bay at Sevastopol, where Moscow's Black Sea Fleet has a base.
Russian President Vladimir Putin requested the use of Russian military forces in Ukraine to settle the situation there. The Russian population and the Crimea-based Black Sea Fleet are threatened by the situation in the country, he said.
Russia’s Federation Council has unanimously approved President Vladimir Putin’s request to use Russian military forces in Ukraine. The move is aimed to settle the turmoil in the split country.
Ukraine's ambassador to the United Nations asked an emergency session of the Security Council on Saturday "to do everything possible now" to stop Russia's "aggression" as its troops took over the strategic Crimea region, AP reported.
But action by the UN's most powerful body appears unlikely. As a permanent member, Russia has veto power and can block the council from adopting any resolution criticizing or sanctioning Moscow.
The United States on Saturday called for the swift deployment of international monitors from the United Nations and the Organization for the Security and Cooperation in Europe (OSCE) to Ukraine to help stem the escalating crisis there.
"Russian actions in Ukraine are violating the sovereignty of Ukraine and threaten international peace and security," US ambassador to the United Nations Samantha Power told an emergency meeting of the UN Security Council on the Ukrainian crisis.
NTJ/HH