Hundreds of pro-Russian separatists have stormed the regional government headquarters in Ukraine's eastern city of Luhansk, unopposed by police, the Ukrainian government says.

Kiev has all but lost control of its police forces in parts of eastern Ukraine, where pro-Russian activists have seized buildings in the region's second biggest city of Donetsk and several smaller towns.

"The regional leadership does not control its police force," Stanislav Rechynsky, an aide to Arsen Avakov, the interior minister, told Reuters news agency on Tuesday. "The local police did nothing."

Rechynsky said the ministry had information that they would next try to take the local television centre.

Video footage showed men, some dressed in green camouflage fatigues and holding shields, walking around what appeared to be the foyer of the government headquarters as hundreds massed outside the building's large wooden doors.

Pro-Russian separatists had previously occupied only the local security services' building in Luhansk, which they took in early April.

Sanctions against Moscow

Earlier on Tuesday, Russia's foreign ministry said the European Union should be "ashamed" for "doing Washington's bidding" by punishing Moscow with sanctions over the Ukraine crisis.

The ministry's comment came hours after the EU imposed asset freezes and travel bans on 15 more names of Russian and pro-Moscow Ukrainian officials, including Russia's deputy prime minister, deputy chairman of state Duma and chief of staff of armed forces.

"Instead of forcing the Kiev clique to sit at the table with southeastern Ukraine to negotiate the future structure of the country, our partners are doing Washington's bidding with new unfriendly gestures aimed at Russia," the foreign ministry said.

"If this is how someone in Brussels hopes to stabilise the situation in Ukraine, it is obvious evidence of a complete lack of understanding of the internal political situation ... and a direct invitation for the local neo-Nazis to continue to conduct lawlessness and reprisals against the peaceful population of the southeast," it said in a statement. "Are they not ashamed?"

Grigory Karasin, the deputy foreign minister, said separately that the United States and EU sanctions were "an absolutely counterproductive, trite measure that would force the already critical situation in Ukraine into a dead end," the state-run RIA news agency reported.

Russia has not yet announced any steps in response to the new sanctions.

Pro-Russians react

Vyacheslav Ponomaryov, the de facto mayor of a separatist-held town Slovyansk, said on Tuesday that he would discuss the release of detained military observers with the West only if the EU dropped sanctions against rebel leaders.

The six observers were in Ukraine under the auspices of the Organization for Security and Cooperation in Europe, a democracy watchdog. They were detained last week after separatists said they had found a Ukrainian spy with them.

Meanwhile, John Kerry, the US secretary of state, told the Wall Street Journal late on Monday that the Ukraine crisis was centred on the motivations of Russia's president, Vladimir Putin.

"You almost feel that he's creating his own reality, and his own sort of world, divorced from a lot of what's real on the ground for all those people, including people in his own country," Kerry said, just hours after the US administration announced another round of economic sanctions on Russian individuals and companies.

The crisis is "an amazing display of a kind of personal reaction to something that just doesn't fit into the lessons learned for the last 60 years or 70 years... It's unfortunate for the Russian people, clearly don't fit into the costs that are being attached to this, it appears to be so personal to President Putin."