Ukraine says Saturday's attacks by pro-Russian militants in eastern Ukraine are "an act of external aggression" by Russia, and says security officials are preparing to implement "an operational response plan."
Interior Minister Arsen Avakov's evaluation appeared on Facebook Saturday, shortly after armed militants with Russian weapons seized police headquarters in the largely ethnic Russian cities of Donetsk and Kramatorsk.
Witnesses, including western journalists, say the Kramatorsk facility was captured after a firefight, but there were no immediate reports of casualties.
The takeover of police facilities in Donetsk prompted the city's police chief to resign, while elsewhere, Western news accounts late Saturday said militants controlled the eastern city of Sloviansk.
Meanwhile, the United States has called on Russia to "cease all efforts" to destabilize Ukraine. A White House National Security Council spokeswoman said Saturday the United States is concerned that Russian separatists -- with apparent support from Moscow -- are "inciting violence and sabotage" against the Ukrainian state.
In Moscow, Russian state television reports Foreign Minister Sergei Lavrov told U.S. Secretary of State John Kerry Saturday that the crisis is caused by the Kyiv government ignoring "the legitimate needs and interests of the Russian and Russian-speaking population" of the region.
Lavrov also warned that any use of force by Kyiv could undermine diplomatic efforts to resolve the crisis.
Moscow has repeatedly denied any role in Ukraine's unrest, which erupted in full two months ago, when anti-Russian protesters in Kyiv forced then-president Viktor Yanukovych to flee the country.
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 ahead of last month's secession referendum.
Days after that vote, the Russian parliament voted to annex the peninsula, prompting the United States and the European Union to impose economic and travel sanctions on Moscow.
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 payment arrears. Early this month, the Russian energy giant Gazprom announced two price increases that effectively raise Ukrainian gas costs by about 80 percent. Additionally, Russian President Vladimir 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. The White House says U.S. Vice President Joe Biden will travel to Kyiv April 22nd.