Ukraine's police and security forces are "helpless" to quell unrest in two eastern regions bordering Russia, and in some cases are cooperating with pro-Russian gunmen who have seized scores of government buildings and taken people hostage, the country's acting president says.

Oleksandr Turchynov said on Wednesday his goal now was to prevent the agitation from spreading to other territories.

"I will be frank: today, security forces are unable to quickly take the situation in the Donetsk and Luhansk regions under control," Turchynov said at a meeting with regional governors.

"The security bodies ... are unable to carry out their duties of protecting citizens. They are helpless in those matters.

"Moreover, some of those units are either helping or cooperating with terrorist organisations," he said.

Turchynov instructed the governors to try to prevent the threat from overtaking more central and southern regions.

"Mercenaries and special units that are active on Ukrainian territory have been tasked with attacking those regions. That is why I am stressing: our task is to stop the spread of the terrorist threat first of all in the Kharkiv and Odessa regions," Turchynov was quoted as saying by the Interfax news agency.

Military drills

Kiev city authorities, meanwhile, announced unexpected middle-of-the night military drills starting on Wednesday night to Thursday morning which will involve military equipment, further stoking tensions.

Russia has placed tens of thousands of troops near the border with Ukraine, and Turchynov said the threat of a Russian invasion was real.

He called for creating regional self-defence units throughout the country, according to Interfax.

Some Ukrainians said they were appalled by the loss of control over eastern regions and accused the central government of inaction.

"In a normal society when Oleksandr Turchynov admits the fact that the authorities do not control the situation in the east of the country is ground for resignation. And not just of him, but all the security forces," Valeriy Kalnysh, former editor of the Kommersant daily, wrote on Facebook. "But can we afford this now? .... And is it the right move in the conditions of an undeclared war with Russia?"

Turchynov spoke hours after pro-Russian gunmen seized more administrative buildings in eastern Ukraine. Insurgents wielding automatic weapons took control and hoisted an insurgent flag on top of the city council building on Wednesday morning in the city of Horlivka in the Donetsk region. They also took control of a police station in the city, adding to another police building which they had controlled for several weeks.

An Associated Press reporter saw armed men standing guard outside the building and checking the documents of those entering.

One of the men said foreign reporters would not be allowed in and threatened to arrest those who did not obey orders. Similar guards were also seen outside the police station in the city.

The pro-Russian separatists now control buildings in about a dozen cities in eastern Ukraine, demanding broader regional rights as well as greater ties or outright annexation by Russia. They are holding some activists and journalists hostage, including a group of observers from a European security organisation.

In Luhansk, one of the largest cities in eastern Ukraine, gunmen in camouflage uniforms maintained control of several government offices they seized on Tuesday.

Eastern Ukraine, which has a large Russian-speaking population, was the heartland of support for Viktor Yanukovych, the ousted president who fled to Russia in February.

The government that replaced him in Kiev has resisted the separatists' demands, fearing they could lead to a breakup of the country or mean that more regions could join Russia, as Crimea did.

Kiev and Western governments accuse Moscow of orchestrating the protests in eastern Ukraine. The United States and the European Union rolled out a fresh set of economic sanctions against Russia this week, but Moscow has

remained unbowed, denying its role in the unrest and saying the actions were Kiev's fault.