Russian troops and military planes were flowing into Crimea on Monday in violation of accords between the two countries, Ukrainian border guards said.
Since Sunday, 10 Russian combat helicopters and eight military cargo planes have landed on the flashpoint Black Sea peninsula, the guards said in a statement, while four Russian warships have been in the port of Sevastopol since Saturday.
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Kiev received no warning regarding the troop movements, even though that is required by the international laws regarding the stationing of Russia's Black Sea navy in Crimea.
Under these agreements, Ukraine should receive notice of any troop movements 72 hours in advance.
Russian fighter jets twice violated Ukraine's air space over the Black Sea during the night on Sunday, Interfax news agency quoted the defence ministry as saying on Monday. It said Ukraine's air force had scrambled a Sukhoi SU-27 interceptor aircraft and prevented any "provocative actions" but gave no further details.
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Pro-Russian demonstrators occupied the first floor of the regional government building in east Ukraine's city of Donetsk on Monday. A Reuters reporter in a press centre on the fourth floor of the building said the protesters had seized the first floor but were unable to go higher because lifts were disabled and stairwell doors shut.
The 11-storey building has been flying the Russian flag, rather than the Ukrainian flag, for three days, with demonstrators carrying Russian flags staging rallies outside.
Crimea, which has housed Russian navies since the 18th century, has come under control of Russian forces and local pro-Kremlin militia, who have surrounded several Ukrainian military bases.
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Ukrainian defence minister Igor Tenyukh has accused Russia of sending 6,000 additional troops into Crimea.
On Saturday, Russia's parliament gave President Vladimir Putin the green light to send troops to Ukraine, in a crisis that threatens to escalate into the worst since the Cold War.
UN Secretary General Ban Ki-moon called on Russia to avoid actions that might worsen the situation in crisis-hit Ukraine. "I urge the Russian Federation to refrain from any acts... that could further escalate the situation," Ban said to the press ahead of a scheduled meeting with Russian foreign minister Sergei Lavrov.
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Video: Ukraine protesters storm the building