Donetsk, Ukraine - Russian-led paramilitaries are fortifying Donetsk’s provincial government building after kicking out separatist administrators, signalling a shift in power as rebels prepare for an expected assault by Ukrainian troops.

The Russian-dominated Vostok Battalion was securing the building on Friday, a day after it removed local separatist fighters and accused them of looting shops while they engaged Ukrainian forces at the city’s international airport earlier this week.

Vostok soldiers set up defences outside of the building, including an anti-aircraft gun, and bulldozed barricades to create a "free-fire zone" to prepare for an expected attack by Ukrainian soldiers.

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The preparations are being made as civilians begin to leave the city, with the rail station packed as rumours circulate of an impending Ukrainian attack on separatist positions.

The president-elect of Ukraine, Petro Poroshenko, promised on Thursday to "punish" the rebels in the east of the country following the downing of a Ukraine helicopter in Slovyansk that killed 14 soldiers, including a general. Poroshenko described the rebels as "terrorists" and "bandits".

The Ukrainian authorities said the helicopter was shot down by a sophisticated surface-to-are missile supplied by Russia.

More than 35 separatist fighters were killed in the battle for the airport, as Ukrainian forces launched helicopter attacks and sent paratroopers to regain control of the airport.

Since then, separatists have brought more fighters and supplies into the city, with a huge convoy driving through the centre of the city on Thursday. Many are believed to have come from Russia.

Al Jazeera witnessed the takeover of the provincial government building on the same day.

Vostok fighters arrested about a dozen separatists inside, whom they accused of looting, and kicked out the rest of the "administration" - mostly poorly clothed, poorly armed and poorly informed locals.

The actions of the Vostok Battalion, under the command of Alexander Khodkovsky, the head of the Donetsk People’s Republic’s security service, appeared to have been part of an internal "coup" under the pretence of restoring order.

Separatists shore up defences

The takeover also allows the Vostok Battalion to secure important sites, manning them with seasoned, well-armed and trained men to prepare for any attack by the Ukrainian army.

On the eighth floor of the building, a Vostok soldier who called himself Sergey broke through a door locked by the previous occupants to find piles of looted goods, including cartons of cigarettes, toothpaste, cheap Chinese-made toys and even kilos of butter.

"While we were fighting at the airport, these guys were looting the Metro supermarket nearby," Sergey said.

"This is not what the Donetsk People’s Republic is about. This is a bad joke."