Warning shots were fired to prevent an unarmed international military observer mission from entering Ukraine's Russian-occupied Crimea yesterday, as new confrontations between Russian and Ukrainian troops raised tension ever higher.

Russia's seizure of the Black Sea peninsula has so far been bloodless, but its forces have become aggressive towards Ukrainian troops, who are trapped in bases and have offered no resistance. 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.

A spokeswoman for the Organisation for Security and Cooperation in Europe (OSCE) said no-one was hurt when shots were fired to turn back its mission of more than 40 unarmed observers, who have been invited to the region by Kiev but do not have permission from Crimea's pro-Russian separatist regional authorities.

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They had been turned back twice before, but this was the first time shots were fired.

Kiev's security council said it had been targeted by hackers in a "massive" denial of service attack designed to cripple its computers. The national news agency was also hit, it said.

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.

The pro-Moscow authorities have ordered all remaining Ukrainian troop detachments in Crimea to disarm and surrender, but at several locations they have refused to yield.

On Friday night, Russian troops drove a truck into a missile defence post in Sevastopol and took control of it. Ukraine's border service said Russian troops also seized a border guard outpost in the east of the peninsula, kicking the Ukrainian officers and their families out of their apartments in the middle of the night.

"The situation is changed. Tensions are much higher now. You have to go. You can't film here," a Russian soldier carrying a heavy machine gun told journalists at a Ukrainian navy base in Novozernoye.

Vadim Filipenko, the Ukrainian deputy commander at the base, said: "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."