By Peter Graff and Andrew Osborn

KIEV/SEVASTOPOL, Ukraine (Reuters) - 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 meters over the regional border.

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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.

A Reuters reporting team filmed a convoy of hundreds of Russian troops in about 50 troop trucks, accompanied by armored 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 harbor'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 defense ministry said it was mobilizing 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 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.

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.

The European Union is also considering sanctions, but has so far been more cautious. Any action would be much harder to organize for a 28-nation bloc that takes decisions unanimously, many of whose members depend on Russian natural gas.

Ukraine's ambassador to Russia held a "frank" meeting with a deputy Russian foreign minister, Moscow said, giving no details.

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.

"MANY HOTHEADS"

"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 another 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.

As tempers have hardened, journalists have been beaten by hostile pro-Russian 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, bused in from southern Russia.

In Crimea, Russian television and the provincial channel controlled by Aksyonov broadcast wildly exaggerated 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.

(Additional reporting by Alissa de Carbonnel in Simferopol, Pavel Polityuk in Kiev and Vladimir Soldatkin in Moscow; Editing by Andrew Roche)