With diplomacy at a standstill, Russia said the United States had spurned an invitation to hold new talks on resolving the crisis, the worst East-West standoff since the Cold War.
Ukrainian Prime Minister Arseny Yatseniuk, who said he would address the U.N. Security Council on Thursday, put the blame for the crisis on Russia and accused Moscow of undermining the global security system by taking control of Crimea.
Russian forces have in little more than a week taken over military installations across Crimea, home to the Russian Black Sea Fleet and Russian territory until Soviet leader Nikita Khrushchev gave it to Ukraine in 1954.
Pro-Russian separatists have taken control of the regional parliament, declared Crimea part of the Russian Federation and announced a referendum for Sunday to confirm this.
President Vladimir Putin says Moscow is acting to protect the rights of ethnic Russians, who make up a majority of Crimea's population, after Ukraine's president Viktor Yanukovich was ousted in what Russia says was an unconstitutional coup.
On Monday, a group of about 10 unidentified armed men fired in the air at a Ukrainian naval post in Crimea, a Ukrainian defense spokesman was quoted as saying.
Ukraine's Channel 5 television quoted Vladislav Seleznyov as saying the shooting took place at a motor pool base near Bakhchisaray. The men in two minibuses drove into the compound and demanded Ukrainian personnel there give them 10 trucks.
Earlier, Interfax Ukraine news agency quoted an unnamed Ukrainian official describing the men as Russian troops and saying that none of the Ukrainians at the site was injured.
Russian forces, who have been in control of Crimea for more than a week, have not so far exchanged fire in anger with Ukrainian troops. Shots were fired over the heads of a group of Ukrainians during a standoff at a military airfield last week.
In other armed action, Russian forces took over a military hospital and a missile unit. Reuters correspondents also saw a big Russian convoy on the move just outside the port city of Sevastopol near a Ukrainian air defense base.
It comprised more than 100 vehicles, including around 20 armored personnel carriers, plus mobile artillery.
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