Pro-Russia separatists stormed or took control of police headquarters and government buildings in several cities in eastern Ukraine on April 12, in what the Ukrainian government called an "act of aggression" by Russia.
Acting Ukrainian Interior Minister Arsen Avakov said the Ukrainian Interior and Defense ministries "are implementing an operational response plan" to apparently coordinated attacks in several cities and towns.
In the city of Kramatorsk, armed men in matching uniforms seized the police headquarters after an apparent gunfight with security forces.
Reuters video footage showed the gunmen, many of them masked, shouting in Russian and firing automatic weapons outside the building before occupying it.
Separatists earlier stormed or took control of police stations and government buildings in the cities of Slovyansk, Donetsk, Krasny Liman, and Kramatorsk.
In Donetsk, the regional police chief has resigned amid pro-Russian protests in the regional capital, Donetsk.
Kostyantyn Pozhydayev announced his resignation on April 12, saying he was forced to step down by pro-Russian activists.
WATCH: Armed Men Seize Police Station In Eastern Ukraine
Some 1,000 protesters surrounded the regional police headquarters in Donetsk on April 12 before storming it.
Reports say armed men now occupy the building.
A meeting of the National Security Council called by acting President Oleksander Turchynov analyzed the situation.
A statement on April 13 said the meeting discussed "issues related to normalizing the situation in eastern Ukraine," without giving any details.
Local media reports said that the meeting discussed the possibility of declaring a state of emergency in the east of the country.
Also on April 12, Turchynov dismissed the chief of the Security Service in the eastern Donetsk region, Valeriy Ivanov.
U.S. Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov that Russia would suffer "additional consequences" if it did not act to de-escalate the situation in eastern Ukraine and pull its troops back from the border.
A U.S. official said the warning came in a phone call with Lavrov on April 12.
Russia's Foreign Ministry said in a statement that Kerry "could not give any concrete facts" to back his concerns.
Lavrov in turn told Kerry that any armed action by Ukrainian authorities in eastern Ukraine would put planned talks at risk.
He said that any use of force against ethnic Russians in the east of Ukraine "would undermine the potential for cooperation ... including the holding of planned four-party talks in Geneva" on April 17.
The Geneva talks are to be attended by Russia, Ukraine, the United States and the European Union.
Lavrov said Ukraine was "demonstrating its inability to take responsibility for the fate of the country."
Earlier on April 12, acting Ukrainian Foreign Minister Andriy Deshchytsya told Lavrov via telephone that Moscow must stop "provocative actions by Russian special agents in Ukraine's eastern regions."
Live Blog: As The Day Unfolded
Lavrov said he rejected all accusations of Russian agents or military troops being in eastern Ukraine and called on Kyiv to stop blaming Moscow for its problems.
Deshchytsya also confirmed his participation in the U.S.-EU-Russia-Ukraine talks in Geneva scheduled for April 17.
But a Russian Foreign Ministry official said there were still many details about the meeting to resolve, including the agenda, before Russia would take part in it.
The Ukrainian Interior Ministry also issued a statement early on April 12 warning public and political organizations to "stop bringing militants onto the streets, stop calling for violence and aggression, stop provoking clashes."
The ministry said people involved in organizing mass disturbances will face jail sentences of between five and eight years.
Separatist protesters first stormed government buildings in the eastern Ukrainian cities of Kharkiv, Donetsk, and Luhansk on April 6.
Only the protesters in Kharkiv have been removed from the regional government building they had occupied.
PHOTOS: Separatist Protesters Take Over Police HQ In Slovyansk
With reporting by Reuters, Interfax, AFP, and ITAR-TASS