Russian President Vladimir Putin warned that Ukraine was "on the verge of civil war" and described the actions of Ukraine's military in the east as "unacceptable".

Putin made the first comment during a phone conversation with German Chancellor Angela Merkel, whereas the second was in a call to Israeli Prime Minister Benjamin Netanyahu, as RIA Novosti reported.

His words were later echoed by the Russian Foreign Minister Sergei Lavrov, who called on Kiev to "be respectful" of the demands of Ukrainian citizens in thе east and south.

Lavrov stressed the need to "immediately start negotiations" with protesters to de-escalate tensions, instead of fomenting unrest by using force and deploying the army, a move which in his words "contradicts Ukrainian law".

Prime Minister Arseniy Yatsenyuk has in turn urged Russia to "stop supporting terrorists".

In a televised address, he argued that Russia wanted to build a "Berlin Wall" in the country.

Conflicting reports make it difficult to confirm the events in the towns of Kramatorsk and Slaviansk, where Ukraine's anti-terrorist operation against what Kiev calls "separatists" was triggered on Tuesday afternoon.

Six armored vehicles apparently passed through Slaviansk, but while some sources argue Russian flags were waving over them, other claim they belong to the Ukrainian army.

In Kramatorsk, the same number of vehicles were reported to have been seized by pro-Russian activists. Ukraine's Defense Ministry has refuted claims they were under Russian command, according to UNIAN news agency.

Earlier the BBC reported that Ukrainian troops had entered Kramatorsk, but were blocked by civilians.

In Donetsk, where a People's Republic was proclaimed last week by pro-Russian separatists, armed men stormed the city council building, according to local media reports.

No violence was observed during the attack, as the rebels met no resistance and were able to easily take control of the building.

Witnesses however suggest that the men took "documents" out of the council. Rebels did not immediately place any demands.

Most protesters in Eastern Ukraine, however, have recently demanded that Ukraine become a federation.

Pro-Russian activists meanwhile declared a "People's Republic" in Odessa, a city in Southern Ukraine.

The so-called "Anti-Maidan" movement urged citizens to disrupt public and private transport across the city and to prevent police and military from stepping in, as RIA Novosti reports.

On its website, the movement said Odessa's population should take these measures at 16:00 local time.

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