DONETSK, Ukraine - Men in the uniforms of Ukraine’s defunct riot police Saturday occupied police headquarters in Donetsk, the eastern city that is one of the flash points of a wave of pro-Russian protests, hours after armed men seized police headquarters and a branch of the security service in a nearby city.

Ukrainian Interior Minister Arsen Avakov described the unrest as “Russian aggression.”

Oleksandr Turchynov, Ukraine’s acting president and parliamentary speaker,called an emergency meeting of the country’s National Defense and Security Council on Saturday night in Kiev to discuss the deteriorating situation in the east. The session lasted through midnight.

“Members discussed anti-terrorist measures and issues related to stabilizing the situation on Ukraine’s east,” according to a statement released on the parliament’s website.

In a phone call with Russian Foreign Minister Sergey Lavrov on Saturday, U.S. Secretary of State John Kerry “expressed strong concern” that the attacks “were orchestrated and synchronized, similar to previous attacks in eastern Ukraine and Crimea,” according to the U.S. State Department.

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 department said.

The Russian news agency Itar-Tass, citing Russia’s Foreign Ministry, said Kerry “could not give any concrete facts” to support his allegations. The news agency said Lavrov told Kerry that the crisis in Ukraine was because of the failure of the Ukrainian government “to take into account the legitimate needs and interests of the Russian and Russian-speaking population.”

Unlike the unidentified armed men who seized Ukrainian government buildings and military facilities in Crimea - and later turned out to be Russian soldiers - in late February and early March as a prelude to Russia’s annexation of the peninsula, the gunmen behind Saturday’s attacks appeared to be Russian-speaking local residents and not professional Russian troops.

The unrest in Donetsk and Slovyansk, about 55 miles to the north, was the latest show of spiraling anger in eastern Ukraine, which has a large Russian-speaking population and was the support base for Viktor Yanukovych, the Ukrainian president who was ousted in February after months of protests in the capital, Kiev. Ethnic Russians in Ukraine’s east widely fear that the authorities who took over after Yanukovych’s fall will suppress them.

Witnesses said the men who entered the police building in Donetsk on Saturday were wearing the uniforms of the Berkut, the riot police squad that was disbanded in February after Yanukovych’s ouster. Berkut officers’ violent dispersal of a demonstration in Kiev in November set off vast protests in the capital that culminated in more bloodshed in February when more than 100 people died in sniper fire, according to the acting government.

It was not immediately clear if the men who occupied the Donetsk police building had made any demands, but the Donetsk police chief said on national television that he was forced to offer his resignation. Interfax Ukraine reported that pro-Russian protesters had invited the former police chief to resume his duties.

Additionally, Donbass News, a local media organization, reported Saturday that the head of the regional branch of Ukraine’s state security service, Valery Ivanov, had been fired by the authorities in Kiev. It gave no reason.

Opponents of the pro-Russian activists in Donetsk have accused the region’s police and security service of sympathizing with calls for secession and of failing to take a robust stand against separatist militants.

In Slovyansk, the mayor said the men who seized the police station were demanding a referendum on autonomy and possible annexation by Russia. Protesters in other eastern cities have made similar demands after a referendum in Crimea last month in which voters opted to split off from Ukraine.

About 20 men in masks and armed with automatic rifles and pistols were guarding the entrance to the police station in the city of about 120,000 people, and another 20 were believed to be inside. They wore St. George’s ribbons, which have become a symbol of pro-Russian protesters in eastern Ukraine. The ribbons were originally associated with the Soviet Union’s victory in World War II.

A masked guard in Slovyansk, who gave his name only as Sergei, said they have “only one demand: a referendum and joining Russia.”

The Interior Ministry said in a statement that the attackers in Slovyansk used tear gas and stun grenades when they stormed the building, injuring three policemen. The attackers’ goal was to seize arms from the police station,authorities said, adding that there were about 40 automatic rifles and 400 pistols as well as ammunition inside.

Some 300 automatic rifles were taken from the Donetsk offices of the state security service after it was briefly taken over by pro-Russian protesters last weekend.

The Interior Ministry reported later Saturday that men from the same group in Slovyansk that took the police station had seized the building of the local security service there.

About 10 men wearing camouflage and carrying automatic rifles set up a makeshift checkpoint at the entrance to the city late Saturday afternoon.

Avakov said in his Facebook post Saturday evening that unknown men also opened fire on a police station in Kramatorsk, a town near Slovyansk, and police were engaged in a gunfight with them. In Krasnyi Lyman, another town in the area, men armed with Russian-made automatic rifles attacked a police station, he said.

Police also prevented unknown people from seizing a police station in Horlivka as Ukraine sent special-forces troops to contain the situation, Avakov said.

In Ukraine’s second-largest city, Kharkiv, about 1,000 pro-Kremlin and pro-government protesters rallied at separate downtown squares Saturday. No clashes were reported. Police detained 70 people traveling by bus to Kharkiv after finding them armed with knives, clubs and Molotov cocktails, the Interior Ministry said on its website.

The Kiev authorities and the United States have accused Russia of fomenting the unrest in the east and seeking to use it as a pretext for sending in troops. Russia has reportedly massed forces in areas near the Ukrainian border.

But Slovyansk Mayor Nelya Shtepa said she held talks with the protesters and they were local residents, not Russians.

“They told me: ‘We don’t have anything against you,’” she said, adding that the men said they “want to be heard, want a dialogue with authorities in Kiev.”

Russia’s Foreign Ministry on Friday warned the Ukrainian government against using force against protesters, saying that such action would derail the talks on settling the crisis between the United States, the European Union, Russia and Ukraine set for this week, as well as any other diplomatic efforts.

Those four entities’ foreign ministers plan to meet Thursday in Geneva for talks, German government deputy spokesman Christiane Wirtz said at a news briefing in Berlin on Friday.

Additionally, U.S. Vice President Joe Biden will visit Ukraine later this month to meet with government and citizen leaders.

The White House said Biden will travel to Kiev on April 22 for a visit that will focus on the international community’s efforts to help stabilize and strengthen Ukraine’s economy.

The White House said Saturday that Biden also will assist Ukraine in its efforts aimed at constitutional overhauls, decentralization, anti-corruption efforts, and free and fair presidential elections set for May 25.

Russian President Vladimir Putin has been ratcheting up pressure on Ukraine, threatening earlier this month to halt gas shipments to the country. A stoppage may also hurt supplies to the rest of Europe, he said.

The European Union plans to help Ukraine pay its gas bill, and there’s “no reason to panic” over shipments of the fuel, European Union Energy Commissioner Guenther Oettinger said in an interview with the Austrian radio station ORF.

In Washington, the International Monetary Fund was wrapping up its spring meetings Saturday, with its board expected to approve by early May a $14 billion to $18 billion loan package to help Ukraine avoid a financial collapse.

The World Bank, the fund’s sister organization, is also offering Ukraine billions in aid, with individual countries preparing bilateral packages, as well.

On Monday, U.S. Treasury Secretary Jacob Lew plans to sign a declaration “moving forward” $1 billion in loan guarantees for Ukraine.

Canada on Saturday widened its sanctions to cover a Crimean oil and gas company and two Crimean officials that it said “bear responsibility for the crisis.” In a statement, the Canadian government also said it would “continue to work with allies and like-minded countries to apply pressure to Russia until it de-escalates the situation in Ukraine.”

Meanwhile, the United States is working to convince Europe of the need to punish Russia more severely for its involvement in Ukraine while at the same time warning Moscow to step back or take more financial hits.

Europe is Russia’s largest trading partner, and any fresh sanctions by the EU must be approved by unanimous vote among its 28 member nations.

President Barack Obama has signed orders that would allow the U.S. to penalize key Russian industries. EU foreign ministers are to meet Monday to decide what additional penalties to impose if Russia continues to ignore the West’s warnings.

The next round of sanctions probably will expand the list of high-ranking Russians whose Western assets have been frozen and are barred from traveling to Europe or the U.S.

U.S. moves against Russia’s energy, metals and mining sectors also have been prepared in what Treasury Undersecretary David Cohen has called “a very powerful yet flexible tool that will allow us to respond quickly and meaningfully as events develop in Ukraine.”

Information for this article was contributed by Peter Leonard, Nataliya Vasilyeva, Jim Heintz, Lara Jakes, Lori Hinnant, Juergen Baetz and Geir Moulson of The Associated Press; by Andrew Higgins and Annie Lowrey of The New York Times; and by Daria Marchak, Volodymyr Verbyany, David McQuaid, John Walcott, Daryna Krasnolutska and Stepan Kravchenko of Bloomberg News.

Front Section, Pages 1 on 04/13/2014