Ukraine says it is launching a "full-scale anti-terrorist operation" against pro-Russian militias in eastern Ukraine.
In a televised speech Sunday, Ukraine President Oleksandr Turchynov vowed Kyiv will not let Russia take over eastern Ukraine after its annexation of the Crimean peninsula last month. He said he will grant amnesty to any pro-Russian separatists who lay down their weapons by Monday.
He accused Moscow of carrying out a war against Ukraine, once part of its Soviet empire.
The Russian foreign ministry immediately branded Ukraine's operation "a criminal order" and called for urgent consideration of it by the U.N. Security Council.
Mr. Turchynov's speech came hours after Ukrainian special forces and pro-Russian militia exchanged gunfire in the eastern Ukrainian city of Slovyansk, with both sides reporting casualties.
Ukraine's Interior Minister Arsen Avakov said a security service officer was killed and another five wounded, the latest skirmish in the aftermath of Moscow's Crimea take-over last month. At least one pro-Russian activist was also killed in the gunfire and two injured.
The escalation of the Ukraine unrest came a day after pro-Russian gunmen took over the Slovyansk police station, and government facilities in the largely ethnic Russian cities of Donetsk and Kramatorsk.
American ambassador to the U.N., Samantha Power, told ABC's "This Week" the unrest in eastern Ukraine "bears the tell-tale signs of Moscow's involvement."
In a statement, NATO chief Anders Fogh Rasmussen said he was "extremely concerned" about the increased tensions in the region. He described it a "concerted campaign of violence by pro-Russian separatists" seeking to destabilize Ukraine.
Fogh Rasmussen called on Russia to "de-escalate the crisis" and pull back thousands of troops from near the Ukrainian border.
On Saturday, U.S. Secretary of State John Kerry, in a telephone call with Russian Foreign Minister Sergei Lavrov, warned there would be additional consequences beyond sanctions already imposed against Russian officials if Moscow did not move to ease tensions in eastern Ukraine.
Lavrov said the crisis was caused by the Kyiv government ignoring the "legitimate needs and interests" of eastern Ukraine's Russian-speaking population.
One pro-Russian protester pleaded with Russian President Vladimir Putin to send troops to support the Russians living in eastern Ukraine.
"We are the residents of the town of Slovyansk, Donetsk region. The National Guard is coming here, there has already been a shootout this morning. We have nothing to hide . Here is my face. I want to ask,'Comrade Putin, you have promised to protect us, please come here and protect us.'"
A White House National Security Council spokeswoman said Saturday the United States is concerned that Russian-speaking separatists - with apparent support from Moscow - are "inciting violence and sabotage" against the Ukrainian state.
Moscow has repeatedly denied any role in Ukraine's unrest, which erupted in full two months ago when then-president Viktor Yanukovych fled the country amid anti-Russian protests in Kyiv.
Despite Moscow's denials, Kyiv and a host of Western governments have cited overwhelming evidence of Russian involvement, including the presence of thousands of Russian troops that infiltrated the Crimean peninsula before last month's secession referendum.
There were other signs of heightened cross-border tensions Saturday, with the Kyiv government saying it is suspending natural gas payments to Moscow. Details of the move were not immediately clear.
Moscow says its neighbor owes $2.2 billion in back payments. Early this month, the Russian energy giant Gazprom announced two price increases that effectively raise Ukrainian gas costs by about 80 percent. Additionally, Russian leader Putin has hinted that Moscow may begin demanding energy payments from Kyiv at the time of delivery.
Top diplomats from Russia, the United States, Ukraine and the European Union are set to hold emergency talks on the crisis April 17 in Geneva. White House officials say U.S. Vice President Joe Biden will travel to Kyiv April 22.