Violence flared in eastern Ukraine on Saturday as pro-Russian gunmen occupied a police headquarters in a small city and attacked government buildings in several towns nearby.

The government convened an emergency meeting late in the day to discuss the unrest, which the country’s acting interior minister said was evidence of “aggression from Russia.”

To Ukrainian officials it looked like the beginning of a replay of the Crimea takeover by Russia, which began with men in unmarked uniforms storming the regional parliament, then spreading their control throughout the peninsula.

Saturday’s action involved only a few dozen men, but the simultaneous assaults in various places — and the modern weapons the men were carrying — suggested a coordinated operation.

“We are very concerned by the concerted campaign we see underway in eastern Ukraine today by pro-Russian separatists, apparently with support from Russia, who are inciting violence and sabotage and seeking to undermine and destabilize the Ukrainian state,” said Laura Lucas Magnuson, a National Security Council spokeswoman, in an e-mailed statement.  “We saw similar so-called protest activities in Crimea before Russia’s purported annexation.  We call on President Putin and his government to cease all efforts to destabilize Ukraine, and we caution against further military intervention.”

A senior State Department official said that during a call Saturday with Russian Foreign Minister Sergei Lavrov, Secretary of State John F. Kerry “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 spokesman said Kerry expressed strong concern to Lavrov that Saturday’s attacks were orchestrated and synchronized and that “militants were equipped with specialized Russian weapons and the same uniforms as those worn by the Russian forces that invaded Crimea.”

The White House said in a statement that Vice President Biden would travel to Kiev this month to meet with government officials and “underscore the United States’ strong support for a united, democratic Ukraine that makes its own choices about its future path.”

In his meetings, which are scheduled for April 22, Biden “will discuss the latest developments in eastern Ukraine,” the statement said.

The main action Saturday took place in the city of Slavyansk, strategically located on a crossroads about 55 miles north of Donetsk and 90 miles from the Russian border. After protesters took over the main police building, they immediately began constructing barricades on roads leading into the city.

Thousands of Russian troops — American and NATO officials put the number at 40,000 — are nearby, conducting a long-running exercise on their side of the border.

And Donetsk has been the scene of a week-long occupation of the regional government headquarters by pro-Russian agitators.

Eastern Ukraine, generally hostile to the new government in Kiev and tied by language and business interests to Russia, has become the focal point of Ukraine’s continuing political crisis. But public support for separatism has been scant, and that may have provoked the more aggressive actions of the past week. Officials in Kiev accuse Russia of fomenting trouble to create a pretext for invasion. Moscow has repeatedly denied that.

Ukraine’s acting foreign minister, Andriy Deschytsia, spoke by phone Saturday with Lavrov and “demanded an end to the provocative activity by Russian special agents in the eastern regions of Ukraine,” the Ukrainian Foreign Ministry said in a statement.

Deschytsia said Russia is trying to disrupt a meeting Thursday in Geneva that is scheduled to bring together representatives of Russia, Ukraine, the European Union and the United States to seek a resolution to the Ukrainian issue. Russia has insisted that the Ukrainian delegation represent a broad sample of Ukrainian opinion, not just the views of the government in Kiev.

Lavrov, in a television interview to be broadcast Sunday, said no Russian agents or troops are in southeastern Ukraine.

An issue facing authorities in Kiev is the reliability of their police forces. The head of the Ukrainian Interior Ministry’s main office in the Donetsk region, Kostiantyn Pozhydayev, met with pro-Russian demonstrators at his headquarters Saturday and announced his resignation.

There were reports that the militants had taken over the building, but the Interior Ministry denied that. The ministry said Saturday evening that its forces had repelled attacks on buildings in the towns of Kramatorsk and Krasny Liman.

In Slavyansk, the mayor, Nelly Shlepa, told the news agency that her city would immediately hold a referendum. Crimea, before it appealed to Russia for annexation, conducted a similar snap vote.

Shlepa said 92 percent of Slavyansk’s business is with Russia, and she referred to Russia as an “older brother.”