Police detained 70 pro-Moscow protesters occupying a regional administration building in eastern Ukraine overnight, but others held out in a standoff in two further cities in what Kiev says is a Russian-led plan to dismember the country.

Ukraine says the seizure of public buildings in its mainly Russian-speaking industrial heartland on Sunday night is a replay of events in Crimea, the Black Sea peninsula Moscow annexed last month.

Interior Minister Arsen Avakov said protesters in the town of Kharkiv had been cleared in a lightning, 18 minute "anti-terrorist" operation, pinning responsibility for the building's occupation on Russian leader Vladimir Putin and Ukraine's ousted Moscow-backed president Viktor Yanukovich.

"All this (in Kharkiv) was inspired and financed by the Putin-Yanukovich group," said Avakov.

NATO warned Moscow of  "grave consequences" to its relationship with the West if it intervened further in Ukraine. But Russian Foreign Minister Sergei Lavrov dismissed U.S. accusations that Moscow was destabilising Ukraine, saying the situation could improve only if the interests of Russian speakers were taken into account.

The pro-Russian protesters have been demanding that referendums be held on whether to join Russia like the vote, dismissed by the Kiev and West as illegal, that endorsed Crimea's return to rule by Moscow.

An aide to Avakov said police went in when the Kharkiv protesters failed to give themselves up and surrender their arms. No shots were fired by the police, although some had been from the other side and a grenade was thrown, he said. One police officer was badly wounded and some others less seriously hurt.

The standoff however continued in the mining center of Donetsk, Yanukovich's home base, where a group of pro-Russian deputies inside the main regional authority building on Monday declared a separatist republic.

Ukraine has been in turmoil since late last year when Yanukovich rejected closer relations with the European Union and tilted the former Soviet republic back towards Moscow. That provoked mass protests in which more than 100 people were killed by police and which drove Yanukovich from office in February, leading to Kiev's loss of control in Crimea.

Police say that in a third protest in the city of Luhansk pro-Russia activists inside the main state security building have seized weapons. There was no clear sign that further police operations were imminent in these two cities. "We hope the buildings occupied in Donetsk and Luhansk will soon be freed," acting president Oleksander Turchinov said.

Numbers of protesters involved appear to be small and Ukrainian nationalists, who believe the operations are being coordinated from Russia, say protesters occupying the buildings have been helped by the inaction of corrupt local police.

In Donetsk, steel-and-energy tycoon Rinat Akhmetov, Ukraine's richest man, is mediating with the protesters, but he may have complicated the plans of the authorities by publicly urging authorities not to use force as a solution.

But authorities may anyway have decided not to tempt action by Moscow and hold back and wait for the protests to fizzle out. The West has expressed concern about what it says has been a buildup of Russian forces along the border with Ukraine. Moscow has said the troops are merely taking part in exercises but NATO Secretary-General Anders Fogh Rasmussen urged caution.

"If Russia were to intervene further in Ukraine it would be a historic mistake," he told a news conference in Paris. "It would have grave consequences for our relationship with Russia and would further isolate Russia internationally."

Lavrov denied responsibility for the trouble in Russian-speaking areas of Ukraine. "One should not seek to put the blame on someone else," he told a new conference in Moscow.

"We are deeply convinced ... that the situation cannot be calmed down and changed into national dialogue if the Ukrainian authorities go on ignoring the interests of the southeastern regions of the country."

On Tuesday about 200 people were gathered in front of the building in Donetsk and a group of National Guard were standing to one side. But the situation was calm and there was no sign of any attempt to enter by force.

In Kharkiv, where the "anti-terrorist" operation was carried out, Ukrainian special forces in combat gear, helmets and balaclavas and carrying machine guns stood guard early outside the building whose outside windows were broken. A partly destroyed sign near the main door read: "Avakov - to jail".

Avakov's ministry said the 70 detained were suspected of "illegal activity related to separatism, the organisation of mass disorder, damage to human health" and breaking other laws.

Turchinov said on Monday that Moscow was attempting to repeat "the Crimea scenario" in which pro-Russian forces seized the local parliament and bulldozed through the referendum, clearing the way for Russia to annex the peninsula.

Unlike in Crimea, where ethnic Russians form a majority, most people in the east and south are ethnically Ukrainian, although they speak Russian as a first language.

U.S. Secretary of State John Kerry told Lavrov in a phone call on Monday that Washington was watching events in eastern Ukraine with great concern and any further moves by Moscow to destabilise Ukraine would "incur further costs for Russia".

Kerry "called on Russia to publicly disavow the activities of separatists, saboteurs and provocateurs" in Ukraine, the State Department said.

The White House warned Putin against moving "overtly or covertly" into eastern Ukraine and said there was strong evidence that pro-Russian demonstrators in the region were being paid.

Keeping up pressure on Kiev today, the Russian foreign ministry called on Ukraine to stop massing military forces it said were tasked with suppressing anti-government protests.

The dispute has raised fears that Russia might restrict its gas supplies to Ukraine's crippled economy. Russian producer Gazprom confirmed Ukraine had failed to pay for its March supplies but did not say whether the company would take any action against Kiev.

Gazprom cut off supplies to Kiev during price disputes in the winters of 2008/2009 and 2005/2006, disrupting the flow of Russian gas to the EU that is carried via Ukraine. Gazprom said gas transit via Ukraine to Europe remained stable today.