Ukraine's authorities plan to launch a "full-scale anti-terrorist operation" involving the army against pro-Russian separatists, acting president Oleksander Turchinov said on Sunday in a televised address to the nation.
Turchinov said Russia was waging a war against Ukraine by sowing disorder in the east of the country, but he offered not to prosecute any militants who gave up their weapons by Monday morning.
Accordingly, Russia's foreign ministry said that such an announcement by the authorities in Kiev is a "criminal order."
The ministry said the West should bring its allies in Ukraine's government under control. "It is now the West's responsibility to prevent civil war in Ukraine," the foreign ministry said in a statement.
The statement also said that Russia would put an urgent discussion of the situation in eastern Ukraine on the agenda of the United Nations Security Council.
Separatist protesters on Sunday seized control of the mayor's office in the town of Mariupol, eastern Ukraine, on the Azov Sea, local media said.
The protesters entered the building following a rally involving about 1,000 people demonstrating in favor of the creation of a separate republic in eastern Ukraine, a local journalist for the newspaper Priazovsky Worker said.
Police did not try to stop them. They entered the offices, took down the Ukrainian flag and were erecting barricades outside the building, the journalist said.
Meanwhile, the interior minister said one Ukrainian state security officer was killed and five others were wounded on the side of government forces in an "anti-terrorist" operation on Sunday against pro-Russian separatist militants in a city in the east.
On the side of the separatists there had been an "unidentifiable number" of casualties during the operation in the town of Slaviansk, the minister, Arsen Avakov, said on his Facebook page.
The minister asked residents in the eastern city of Slaviansk to stay indoors, in anticipation of clashes between pro-Russian militants who have seized official buildings and Ukrainian security forces.
On another note, UN Secretary General Ban Ki-moon called Saturday on all sides in the Ukraine to “exercise maximum restraint” and engage in dialogue to de-escalate the situation there.
A UN statement said Ban is “deeply concerned” about the deteriorating situation and “growing potential for violent clashes.”
Meanwhile, the White House said that Vice President Joe Biden would travel to Kiev this month to show support for the Ukrainian government as U.S. officials expressed new concern over "violence and sabotage" by militants it said were apparently supported by Russia in eastern Ukraine.
Biden, set to travel to Kiev on April 22, will become the most senior U.S. official to visit the country since the crisis began there.
Secretary of State John Kerry spoke by telephone with Russian Foreign Minister Sergei Lavrov, the State Department said.
Kerry during the call "expressed strong concern that attacks today by armed militants in eastern Ukraine were orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea," said a senior State Department official, speaking on condition of anonymity.
"Militants were equipped with specialized Russian weapons and the same uniforms as those worn by the Russian forces that invaded Crimea. The secretary made clear that if Russia did not take steps to de-escalate in eastern Ukraine and move its troops back from Ukraine's border, there would be additional consequences," the official added.
The official did not state what those consequences would be.
The White House warned Russia against further military action in Ukraine after armed separatists seized government offices in the eastern city of Slaviansk.
In Moscow, the Russian Foreign Ministry said in a statement that Lavrov told Kerry that any armed action by Ukrainian authorities in the east of Ukraine would undermine efforts at a diplomatic solution to the conflict and put planned peace talks at risk. Lavrov said Ukraine was "demonstrating its inability to take responsibility for the fate of the country," according to the Foreign Ministry statement.
Kerry visited Ukraine in March to show support for Ukraine's leaders after Moscow took control of the Crimea region.
In turn, US Ambassador to the UN Samantha Power said that the United States is prepared to step up sanctions against Russia if the recent actions in Ukraine continue.
NATO described the appearance in eastern Ukraine of men with specialized Russian weapons and identical uniforms without insignia - as previously worn by Moscow's troops when they seized Crimea - as a "grave development."
Power said on ABC's "This Week" the latest events in Ukraine bore "the telltale signs of Moscow's involvement."
"I think we've seen that the sanctions can bite and if actions like the kind we've seen over the last few days continue, you're going to see a ramping up of those sanctions," Power said.