The crisis in Ukraine escalated Saturday, as pro-Russian masked gunmen seized a police station and a security-services building in the city of Sloviansk, near the Russian border.

Kyiv Post, a Ukrainian English-language news organization, said the takeover appeared to be a Russian military operation similar to the invasion of Crimea last month, involving men in fatigues carrying Kalashnikov assault rifles. Gunmen reportedly established checkpoints outside the city of 125,000, and appeared to be searching vehicles for weapons, Kyiv Post reported.

Russia supporters also reportedly tried to take the police station in Shakhtarsk, another city in the Donetsk region of eastern Ukraine, that effort apparently failed. In addition, a group of young people armed with wooden bats briefly took over a floor of the general prosecutor's office in the city of Donetsk, Reuters reported. They later left after talks, Donetsk police said in a statement.

Later in the day, the Interior Ministry in Donetsk was also seized by gunmen, Kyiv Post reported. It said the swiftness of the attacks suggested “that operation was planned well in advance and had support from within the ranks of Ukraine's government, including its law enforcement agencies.”

Russia-based media outlet RT published this video of the police headquarters takeover in Sloviansk, and the hoisting of a Russian flag over its entrance:

Another video showed the mayor of Sloviansk, Nelya Shtepa, addressing the crowd that had gathered near the police headquarters. An account by Russia’s Interfax news agency said Shtepa assured residents that the takeover posed no threat to them. The occupiers are on the people's side, she reportedly said; they are showing their disagreement with the Kiev authorities.

In addition, several reports emerged of military jets flying over cities in eastern Ukraine.

Donetsk too. #Ukraine RT @SimonOstrovsky Fighter jet just buzzed Luhansk — Alexander Marquardt (@MarquardtA) April 12, 2014

The seizure of buildings by pro-Russian separatists ratchets up East-West tension over the fate of Ukraine. Earlier this week, NATO released images showing a significant buildup of Russian military equipment and troops along the Ukraine border.

NATO officials said there are 40,000 Russian troops amassed along the border, raising fears of an imminent invasion on a scale that would dwarf Moscow’s takeover of Crimea.

Comparisons with Crimea resonated on Saturday, as correspondents and other observers noted that the gunmen who took over the police station in Sloviansk wore similar camouflage uniforms, and carried the same weaponry as those who occupied Crimea.

In addition, several journalists reported being harassed and roughed-up by the gunmen, as other reporters had been during the Crimea takeover.