Ukraine says Saturday's attacks by pro-Russian militants in eastern Ukraine are "an act of external aggression" by Russia, and 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 more government buildings in the Russian-speaking east, including police headquarters in 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.
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.
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.
Secretary of State John Kerry spoke by telephone with Russian Foreign Minister Sergei Lavrov, the State Department said.
"During the call Kerry 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.
US officials met Friday with Ukrainian finance officials to discuss a "range of strategic and economic issues" according to a State Department statement.
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.
At the Donetsk police headquarters, the Russian colors fly alongside the regional separatist flag.
Around 200 pro-Russian protesters armed with clubs and knives took over the building Saturday and met no resistance -- forcing the police chief to resign.
“Here in Donetsk, everyone is sick of the powers in Kyiv,” complained Oleksander Korfman, one of the organizers. “That includes the police. They can’t break their pledge to the state, but they are with the people.”
Later around a dozen heavily armed Ukrainian police -- known as Berkut -- were welcomed into the headquarters. It appears there is much sympathy for the protesters among security forces.
Most of the protesters then left the police building in the hands of special forces. One protester -- who did not want to be named -- said the takeover was aimed at ensuring police weapons are not used against the demonstrators.
“The special forces will secure the place so no one steals the weapons inside. We will go back to our positions at the regional administration building," the protester said.
Armed pro-Russian groups also stormed police buildings in the nearby city of Slovyansk Saturday. There were reports that the demonstrators seized around 400 firearms. One who gave the name Sergei explained their actions.
“Our people want to live quietly and peacefully, without the junta who seized power in Kyiv," he said, "and so that we are not under America and the West. We don't want to be their slaves. We want to be with Russia.”
Building by building, the protesters are gaining ground. Their challenge to Kyiv's authority is so far unanswered; the dangers of conflict are growing.
NATO says Russian armed forces are massing on Ukraine's eastern border, while Moscow says they are on normal maneuvers.
Ukraine suspends gas payments
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.
Some information for this report provided by Reuters
Images from Ukraine
A pro-Russian protester whose helmet reads "Donetsk Republic," pickets a building where Ukraine's Prime Minister Arseniy Yatsenyuk was meeting with regional leaders from eastern Ukraine, in Donetsk, April 11, 2014.
Leaders of pro-Russian protesters hold a news briefing inside the seized office of the SBU state security service in Luhansk, in eastern Ukraine, April 11, 2014.
A group of pro-Russian activists warm themselves at a bonfire next to barricades in front of an entrance to the Ukrainian regional office of the Security Service in Luhansk, Ukraine, April 11, 2014.
Motorists watch a Ukrainian military convoy pass by near the city of Donetsk, Ukraine, April 10, 2014.
Pro-Russian protesters set barbed wire on a barricade outside the SBU state security service in Luhansk, Ukraine, April 10, 2014.
Masked pro-Russian activists guard barricades at the regional administration building in Donetsk, Ukraine, April 9, 2014.
A pro-Russian activist speaks to other protesters at barricades in front of a security service regional office in Luhansk, Ukraine, April 9, 2014.
A masked man stands in front of barricades and Soviet era red and Russian national flags at an entrance to the regional office of the security service in Luhansk, Ukraine, April 9, 2014.
Communist lawmakers scuffle with right-wing Svoboda (Freedom) Party lawmakers during a session of the Ukrainian parliament in Kyiv, April 8, 2014.
A view through a broken window of the regional administration building shows a cordon of Interior Ministry members blocking a group of pro-Russian protesters in Kharkiv, Ukraine, April 8, 2014.
Street cleaners sweep away bullet cases as they remove trash and a barricade erected by pro-Russian protesters near a building of the state security service in Donetsk, Ukraine, April 8, 2014.
Pro-Russian activists gather behind a barricade with Russian flags in front of the entrance to the regioanl security service office in Luhansk, west of the Russian border, in Ukraine, April 8, 2014.