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Warning shots fired to turn monitors back from Crimea | 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.

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.

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.