Shots were fired in Crimea to warn off an unarmed international team of monitors and at a Ukrainian observation plane, as the standoff between occupying Russian forces and besieged Ukrainian troops intensified.
Russia's seizure of the Black Sea peninsula, which began 10 days ago, has so far been bloodless, but its forces have become increasingly aggressive towards Ukrainian troops, who are trapped in bases and have offered no resistance.
President Vladimir Putin declared a week ago that Russia had the right to invade Ukraine to protect Russian citizens, and his parliament has voted to change the law to make it easier to annex territory inhabited by Russian speakers.
Tempers have grown hotter in the last two days, since the region's pro-Moscow leadership declared it part of Russia and announced a March 16 referendum to confirm it.
The worst face-off with Moscow since the Cold War has left the West scrambling for a response. U.S. Secretary of State John Kerry, speaking to Russia's foreign minister for the fourth day in a row, told Sergei Lavrov that annexing Crimea "would close any available space for diplomacy," a U.S. official said.
President Barack Obama spoke by phone to the leaders of France, Britain and Italy and three ex-Soviet Baltic states that have joined NATO. He assured Latvia, Lithuania and Estonia, which have their own ethnic Russian populations, that the Western military alliance would protect them if necessary.
A spokeswoman for the Organization for Security and Cooperation in Europe said no one was hurt when shots were fired to turn back its mission of more than 40 unarmed observers, who have been invited by Kiev but lack permission from Crimea's pro-Russian authorities to cross the isthmus to the peninsula.
They had been turned back twice before, but this was the first time shots were fired.
Ukraine's border guards said an unarmed observation plane took rifle fire flying 1,000 metres over the regional border.
Hackers targeted Kiev's security council with a denial of service attack designed to cripple its computers, the council said. The national news agency was also hit. Russia used similar cyber tactics during its war against Georgia in 2008.
Crimea's pro-Moscow authorities have ordered all remaining Ukrainian troop detachments in the province to disarm and surrender, but at several locations they have refused to yield.
Moscow denies that the Russian-speaking troops in Crimea are under its command, an assertion Washington dismisses as "Putin's fiction". Although they wear no insignia, the troops drive vehicles with Russian military plates.
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A Reuters reporting team filmed a convoy of hundreds of Russian troops in about 50 troop trucks, accompanied by armoured vehicles and ambulances, which pulled into a military base north of Simferopol in broad daylight on Saturday.
'Situation has changed'
The military standoff has remained bloodless, but troops on both sides spoke of increased agitation.
"The situation is changed. Tensions are much higher now. You have to go. You can't film here," said a Russian soldier carrying a heavy machinegun, his face covered except for his eyes, at a Ukrainian navy base in Novoozernoye.
About 100 armed Russians are keeping watch over the Ukrainians at the base, where a Russian ship has been scuttled at the harbour's entrance to keep the Ukrainians from sailing out with three ships of their navy.
"Things are difficult and the atmosphere has got worse. The Russians threaten us when we go and get food supplies and point their guns at us," said Vadim Filipenko, the Ukrainian deputy commander at the base.
A source in Ukraine's defence ministry said it was mobilising some of its military hardware for a planned exercise, Interfax news agency reported.
Ukraine's military, with barely 130,000 troops, would be no match for Russia's. So far Kiev has held back from any action that might provoke a response.
Overnight, Russian troops drove a truck into a missile defence post in Sevastopol, the home of both their Black Sea Fleet and the Ukrainian navy, and took control of it. A Reuters reporting team at the scene said no one was hurt.
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Ukraine's border service said Russian troops had also seized a border guard outpost in the east of the peninsula overnight, kicking the Ukrainian officers and their families out of their apartments in the middle of the night.
Polish Foreign Minister Radoslaw Sikorski said on Saturday Poland had evacuated its consulate in Sevastopol due to "continuing disturbances by Russian forces".
The United States has announced sanctions against individuals it accuses of interfering with Ukrainian territorial integrity, although it has yet to publish the list. Washington has threatened wider action to isolate the Russian economy.