Russia called for a meeting of the United Nations Security Council over a Ukrainian army operation in the south-eastern city of Slaviansk, Russian news agencies said on Friday, quoting Russia’s mission to the U.N.

Russia called for a meeting of the United Nations Security Council over a Ukrainian army operation in the south-eastern city of Slaviansk, Russian news agencies said on Friday, quoting Russia's mission to the U.N.

In the southern port of Odessa, one man was shot dead in clashes between supporters of Ukrainian unity and pro-Russian activists, police said. Protesters threw petrol bombs, paving stones and explosive devices during the disturbances.

Though Ukrainian forces embarked on one of their most concerted military operations yet, their advance on the ground in the east was limited.

Nevertheless, President Vladimir Putin's spokesman accused Kiev of firing on civilians from the air in a "punitive operation" that destroyed an international peace plan.

Russia was "extremely worried" about the fate of Russians in the eastern Ukrainian town, including an envoy sent to help free German and other foreign hostages, the Kremlin spokesman said.

Moscow has tens of thousands of troops massed on the border and claims the right to invade if needed to protect Russian speakers.

Ukraine's acting president said Russian "armed saboteurs" had tried to cross into the country overnight, but were pushed back by Ukrainian border troops. He gave no further details.

His Western-backed government rejected Russia's interpretation of events, saying Moscow was supporting groups in eastern Ukraine who were "putting civilians in danger, seizing hostages and creating an atmosphere of terror and violence".

Reuters journalists in Slaviansk, the most heavily fortified bastion of pro-Russian separatists in eastern Ukraine, heard shooting break out and saw one helicopter opening fire before dawn. Ten hours later, the city was largely quiet, with shops shut and armed separatists in control of the streets.

Advancing Ukrainian forces in armored vehicles took up positions closer in to the suburbs, but rebels still controlled most of the town of 130,000.

Acting President Oleksander Turchinov said the operation had been complicated by the rebels' use of human shields and had not progressed as quickly as had been hoped.

The separatist pro-Russian militants had made more moves on Thursday, seizing a rail control centre for the Donetsk region, a railway official said. By cutting off power, they had all but paralyzed train traffic.