Troops sent by Kiev as part of an "anti-terrorist" operation in eastern Ukraine have made progress retaking seized buildings in the port of Mariupol.

Wednesday's developments follow the entering of 10 army tanks and armored vehicles into Mariupol, which was revealed by witnesses cited by local media.

Ukrainian government forces have managed to regain control of the city hall in the south-eastern port after it had been occupied for days by pro-Russian separatists.

The soldiers also started to clear the barricades set up by activists on the central square, according to the BBC.

Activists inside the building seem to have left it, but conflicting information is provided on whether they left the city hall peacefully or showed resistance. Ukraine's Interior Ministry later confirmed that the hall had beed retaken by Kiev forces.

UNIAN news agency reveals that shots could be heard and tyres were set on fire across Mariupol overnight. Slaviansk, another location in the region, was earlier reported to be besieged by Ukrainian forces. Accounts of pro-Russian rebels quoted by ITAR-TASS suggest Kiev soldiers are preventing locals from leaving the town and shoot at their vehicles.

Slaviansk separatists also say that members of the Right Sector movement have appeared at checkpoints leading into the town, but the information could not be independently verified.

At least 30 separatists have been reportedly killed in Slaviansk by special forces over the last 24 hours.

Pro-Russian separatists are still holding their control on a number of government buildings across eastern and southern Ukraine, demanding that the Kiev interim leadership resign. In cities like Donetsk, activists have proclaimed a "People's Republic" and announced they will hold a referendum whether to join Russia on Sunday, May 11.

Interim President Oleksandr Turchynov said last week Ukrainian troops were "helpless" in rebel-controlled areas, but the operation against them has now taken a turn.

The West and Russia accuse each other of fomenting tensions in Ukraine, and Kiev believes Moscow is behind the unrest in Donetsk, Luhansk and Kharkiv regions.

EU foreign policy chief Catherine Ashton and US Secretary of State John Kerry have announced that sanctions against Russia will be toughened if regional polls on breaking away from Ukraine take place as planned.

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