Warning shots were fired to prevent an unarmed international military observer mission from entering Ukraine's Russian-occupied Crimea on Saturday, as new confrontations between Russian and Ukrainian troops raised tension ever higher.
Russia's seizure of the Black Sea peninsula, which began about 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.
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 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 do not have permission from Crimea's pro-Russian separatist regional authorities.
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.
Overnight, Russian troops drove a truck into a missile defense 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.
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.
"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 Novozernoye.
About 100 armed Russians are keeping watch over the Ukrainians at the base, where a Russian ship has been scuttled at the entrance to keep the Ukrainians from sailing out.
"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.
Polish Foreign Minister Radoslaw Sikorski said on Saturday Poland had evacuated its consulate in Sevastopol due to "continuing disturbances by Russian forces".
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 and identify themselves as Russian troops to the besieged Ukrainian forces.
A Reuters reporting team saw a convoy of hundreds of Russian troops in about 50 troop trucks, accompanied by armored vehicles and ambulances, pull into a military base near Simferopol on Saturday.
The United States has announced sanctions against individuals it blames for interfering with Ukrainian territorial integrity, although it has yet to publish the list.
The European Union is also considering sanctions, but this may be much harder to organize for a 28-member bloc that must take decisions unanimously and depends on Russian natural gas.
Russian Foreign Minister Sergei Lavrov gave no indication of any softening of Moscow's position on Saturday, insisting that the government in Kiev had been installed in an illegal coup.
Pro-Moscow Crimea leader Sergei Aksyonov said the referendum on union with Russia - due in a week - would not be stopped. It had been called so quickly to avert "provocation", he said.
"There are many hotheads who are trying to create a destabilized situation in the autonomous republic of Crimea, and because the life and safety of our citizens is the most valuable thing, we have decided to curtail the duration of the referendum and hold it as soon as possible," he told Russian television.
Aksyonov, whose openly separatist Russian Unity party received just four percent of the vote in Crimea's last parliamentary election, declared himself provincial leader 10 days ago after armed Russians seized the parliament building.
Crimean opposition parliamentarians say most lawmakers were barred from the besieged building, both for the vote that installed Aksyonov and the one a week later that declared Crimea part of Russia, and the results were falsified. Both votes took place behind closed doors.
Crimea has a narrow ethnic Russian majority, but it is far from clear that most residents want to be ruled from Moscow. When last asked in 1991, they voted narrowly for independence along with the rest of Ukraine. Western countries dismiss the upcoming referendum as illegal and likely to be falsified.
Many in the region do feel deep hostility to Kiev, and since Aksyonov took power supporters of union with Moscow have controlled the streets, waving Russian flags and chanting "Rossiya! Rossiya!".
Nevertheless, many still quietly speak of their alarm at the Russian takeover: "With all these soldiers here, it is like we are living in a zoo," said Tatyana, 41, an ethnic Russian. "Everyone fully understands this is an occupation."
The region's 2 million population includes more than 250,000 indigenous Tatars, who have returned only since the 1980s after being deported en masse to distant Uzbekistan by Stalin. They are fiercely opposed to Russian annexation.
The referendum is "completely illegitimate. It has no legal basis," Crimean Tatar leader Refat Chubarev told Germany's Suddeutsche Zeitung newspaper. "Different groups of people with different histories live here. A mathematical majority cannot express the wishes of the population."
Journalists have been attacked by hostile crowds. The Associated Press said armed men had confiscated TV equipment from one of its crews.
In addition to the Russian troops, the province is prowled by roving bands of "self-defense" forces and Cossacks in fur hats armed with whips, who were bused in from southern Russia.
Russian television and the provincial channel controlled by Aksyonov broadcast relentless accounts of "fascists" in control of the streets in Kiev and of plans by Ukraine to ban the Russian language. Ukrainian television and the region's only independent station have been switched off.
Putin launched the operation to seize Crimea within days of Ukraine's pro-Russian President Viktor Yanukovich's flight from the country. Yanukovich was toppled after three months of demonstrations against a decision to spurn a free trade deal with the European Union for closer ties with Russia.
The final week of protests saw around 100 people killed in the streets, many shot by sharpshooters on rooftops.
Ukraine is near bankruptcy, and the West has had to step in to replace $15 billion in loans that Putin had promised to Yanukovich. The European Commission has said it could give Ukraine up to 11 billion euros ($15 billion) in the next couple of years provided it reaches agreement with the IMF, which requires economic reforms that Yanukovich had resisted.
Promises of billions of dollars in Western aid for Kiev, and the perception that Putin has held back from ordering troops beyond Crimea into other parts of Ukraine, have helped reverse a rout of the local hryvnia currency.
Russian gas monopoly Gazprom said Ukraine had not paid its $440 million gas bill for February, bringing its arrears to $1.89 billion, and hinted it could turn off the taps.
In Moscow, a huge crowd gathered near the Kremlin on Friday at a government-sanctioned rally and concert billed as being "in support of the Crimean people." Pop stars took to the stage and demonstrators held signs with slogans such as "Crimea is Russian land" and "We believe in Putin."