Unidentified armed men fired in the air as they moved into a Ukrainian naval post in Crimea on Monday in the latest confrontation since Russian military groups seized control of the Black Sea peninsula.
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 defence 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 defence base.
It comprised more than 100 vehicles, including around 20 armoured personnel carriers, plus mobile artillery.
Chance of talks spurned
Putin says Russia is not controlling events in Crimea but denials of Russian involvement are ridiculed by the United States as the two former Cold War enemies wage a geopolitical battle over the future of Crimea and Ukraine.
Russian Foreign Minister Sergei Lavrov told Putin that Russia's position on Ukraine remained at odds with the West, but U.S. Secretary of State John Kerry had declined an invitation to visit Russia on Monday for further talks.
"It is all being formulated as if there was a conflict between Russia and Ukraine ... and our partners suggested using the situation created by a coup as a starting point," Lavrov told Putin during talks in the Black Sea resort of Sochi.
He did not say why Kerry had postponed the talks.
In Kiev, Yatseniuk said he would address the U.N. Security Council during a debate on Ukraine. He is also due to hold talks with the U.S. government which will show Washington's support of the new Ukrainian leadership.
"Russia's policy is aimed at undermining the basis of the global security system and revising the outcome of World War Two," Interfax quoted Yatseniuk as telling reporters.
Ukraine's crisis was triggered in November by Yanukovich's refusal, under Russian pressure, to sign deals on closer political and trade ties with the European Union.
Although three months of protests against Yanukovich were mostly peaceful, at least 80 demonstrators were killed in clashes after police used force against them, some by sniper fire.
Yanukovich fled Ukraine before a peace deal with the opposition was implemented, and a new national unity government was installed. He is wanted for mass murder in Ukraine and is being sheltered by Russia.
West does not recognise referendum
Western countries have denounced the Russian intervention in Crimea and say the borders of Ukraine, a country of 46 million, should remain unchanged. They have said they will not recognise the outcome of Sunday's referendum.
"The United States is not prepared to recognise any result of the so-called referendum taking place in six days time," U.S. Ambassador Geoffrey Pyatt said in Kiev. "We are committed to Crimea's status as part of Ukraine. The crisis needs to be solved diplomatically, not militarily."
In the latest military movements, in the port of Sevastopol, where Russia has its Black Sea Fleet base, Russian forces disarmed servicemen at a Ukrainian army missile base, Seleznyov said.
He told Fifth Channel television that about 200 soldiers aboard 14 trucks moved on the building at about 1.30 a.m and threatened to storm it if the Ukrainian soldiers failed to give up their weapons.
In the eastern city of Luhansk, Ukraine's security services said they were investigating the takeover on Sunday of the main administrative building. The region's top official was held captive in a room where he was made to write a letter saying he had resigned but later said he was still performing his duties.