Update as of 4:05 a.m. EDT: Ukrainian authorities claim they have seized back the regional administration building in Kharkiv, which was under the control of pro-Moscow protesters since the weekend, and said that other government buildings occupied by the rebels in the cities of Luhansk and Donetsk would also be freed soon, BBC reported.

Ukrainian President Oleksandr Turchynov said that several policemen had been hurt in the process of freeing the Kharkiv building and added that people who had taken over the government buildings would be treated as “terrorists and criminals.”

Meanwhile NATO Secretary-General Anders Fogh Rasmussen warned Russia of "grave consequences" if it moved into eastern Ukraine.

"If Russia were to intervene further in Ukraine it would be a historic mistake," he said, at a conference in Paris on Tuesday, according to Reuters, adding: "It would have grave consequences for our relationship with Russia and would further isolate Russia internationally."

Update as of 1:15 a.m. EDT: Ukrainian interior minister Arsen Avakov said Tuesday that the country has launched an "anti-terrorist" operation in Kharkiv and arrested nearly 70 "separatists," Reuters reported.

"An anti-terrorist operation has been launched. The city center is blocked along with metro stations. Do not worry. Once we finish, we will open them again," Avakov reportedly said, on his Facebook page.

Supporters of Russia in the eastern Ukrainian cities of Donetsk and Kharkiv declared their region independent from Ukraine on Monday, escalating concerns in Kiev that Ukraine could be the target of another aggressive expansionist move from Russia, news reports said.

Activists in Donetsk who have seized government buildings in the city since the weekend, announced the formation of the Donetsk People’s Republic, and asked for a Crimea-like referendum to legitimize its secession from Ukraine. In Kharkiv, another Ukrainian city close to the Russian border, Russian supporters violently protested against the present Ukrainian administration and proclaimed a “sovereign Kharkiv People’s Republic,” Associated Press, or AP, reported citing Interfax, a local news agency.

"An anti-crisis headquarters was set up tonight, and anti-terrorist measures will be taken against those who took up arms," Ukraine’s acting President Oleksandr Turchynov said, in a televised address in Kiev on Monday, according to Interfax.

Following Crimea’s annexation by Russia, other Ukrainian cities with a significant Russian-speaking population, such as Donetsk, Kharkiv and Luhansk, have witnessed protests with pro-Moscow activists clashing with Ukrainian forces. In Kharkiv, two people were reportedly injured in clashes between supporters of Russia and those in favor of the current Ukrainian administration, which came to power after unseating former pro-Moscow President Viktor Yanukovych.

Ukraine's Prime Minister Arseniy Yatsenyuk accused Russia of exploiting the unrest as a pretext for sending troops into the country to take over more parts of Ukraine.

"The plan is to destabilize the situation. The plan is for foreign troops to cross the border and seize the country's territory, which we will not allow," Yatsenyuk said, according to AP.

The Russian foreign ministry denied the accusations but warned the Ukrainian government not to resort to any force in response to the people’s “legitimate demands,” stating: "If the political forces that call themselves the Ukrainian government continue to take an irresponsible attitude to the fate of the country and its people, Ukraine will inevitably face new difficulties and crises."

In an op-ed in The Guardian on Monday, Russian Foreign Minister Sergei Lavrov blamed "extremists and neo-Nazis" for spreading chaos in Ukraine and added that, "Russia is doing all it can to promote early stabilisation in Ukraine."

Meanwhile, Jay Carney, spokesperson for the White House, which has condemned Russia’s annexation of Crimea and sought to use sanctions against the country's officials as a punitive measure, said that there was strong evidence that some of the pro-Russian protesters were hired hands and were not residents of Ukraine.

The White House also announced that Secretary of State John Kerry will meet with senior diplomats from Russia, Ukraine and the European Union within the next 10 days to ease tensions in the region, in one of the first four-way talks since the crisis began.