Ukraine has launched an "anti-terrorist" operation in the southeastern city of Kharkiv and has arrested about 70 "separatists" for seizing the regional administration building, Ukrainian Interior Minister Arsen Avakov has said.
Avakov posted on his Facebook page on Tuesday: "An anti-terrorist operation has been launched. The city centre is blocked along with metro stations. Do not worry. Once we finish, we will open them again."
Ukraine's Interior Ministry was quoted as saying by Interfax-Ukraine news agency that those detained were suspected of "illegal activity related to separatism, the organisation of mass disorder, damage to human health" and breaking other laws.
In response, Russia's foreign ministry issued a statement calling on Kiev to stop the operation, warning that it "could lead to an outbreak of civil war".
Meanwhile in Paris, NATO chief, Anders Fogh Rasmussen, warned Russia against intervening further in Ukraine, urging Moscow to "step back".
"Russia's illegal aggression against Ukraine is the greatest challenge to Europe's security in a generation," Anders Fogh Rasmussen told reporters.
"I urge Russia to step back and not escalate the situation in east Ukraine," he said during a seminar on NATO reforms.
"If Russia were to intervene further in Ukraine it would be a historic mistake. It would have grave consequences for our relationship with Russia and it would further isolate Russia internationally," Rasmussen said.
Earlier, acting Ukrainian President Oleksandr Turchynov had pledged that "anti-terrorist measures" would be taken against protesters who had armed themselves.
Turchynov said the action in Kharkiv, Donetsk and Luhansk showed that Russia was "playing out the Crimean scenario" - a reference to the Russian takeover and annexation of the peninsula last March.
The developments prompted the US to warn Russia to stop further destabilising efforts in Ukraine after pro-Russia protesters declared an independepent republic in the region of Donetsk.
White House spokesman Jay Carney said the US was ready to impose further sanctions against Russia should the situation escalate, adding there was strong evidence that some of the protesters "were paid and were not local residents".
At the US State Department, spokesman Jen Psaki said Secretary of State John Kerry had told his Russian counterpart Sergei Lavrov that the demonstrations "do not appear to be a spontaneous set of events".
"Rather, the Secretary noted that this appears to be a carefully orchestrated campaign with Russian support," she said.
Meanwhile, Ukraine's former prime minister and now presidential candidate Yulia Tymoshenko has arrived in Donetsk, and accused the protesters who had captured government buildings of working for Russia's secret services.
On Monday, pro-Russian protesters occupying a regional administration building in Donetsk declared the creation of a "people's republic" separate from Ukraine.
"In the event of aggressive action from the illegitimate Kiev authorities, we will appeal to the Russian Federation to bring in a peacekeeping contingent," said the proclamation, voiced by an unidentified protester.
The activists later read the text to a cheering crowd of about 1,000 people outside the building.
Police in Luhansk also said protesters occupying the state security building there had seized weapons.
Russia said the Ukrainian government should stop blaming Russia for its problems.
Russia has been pushing a plan of a federal Ukraine, in which regions of the country of 46 million would have broad powers of autonomy.
Ukraine, which is drawing up its own plan for "de-centralisation" linked to regional development, says Russia aims to break up the country.