Ukrainian police officers and pro-Russian protesters stand inside the seized regional Interior Ministry building in Donetsk, eastern Ukraine. Pro-Russian gunmen occupied a police headquarters in a small city north of Donetsk and attacked government buildings in several nearby towns.

April 12, 2014 Ukrainian police officers and pro-Russian protesters stand inside the seized regional Interior Ministry building in Donetsk, eastern Ukraine. Pro-Russian gunmen occupied a police headquarters in a small city north of Donetsk and attacked government buildings in several nearby towns. Photomig via European Pressphoto Agency

Pro-Russian gunmen occupied a police headquarters in a small city north of Donetsk, eastern Ukraine, and attacked government buildings in several nearby towns. Pro-Moscow demonstrators have occupied government buildings in Donetsk and Luhansk, another eastern Ukrainian city, since Sunday.

Pro-Russian activists are taking to the streets in eastern Ukraine as the nation’s beleaguered prime minister accuses Russia of plotting takeovers of government buildings in the region. Parts of eastern Ukraine wish to split from Kiev and join Russia.

Pro-Russian activists are taking to the streets in eastern Ukraine as the nation’s beleaguered prime minister accuses Russia of plotting takeovers of government buildings in the region. Parts of eastern Ukraine wish to split from Kiev and join Russia.

Pro-Russia demonstrators in eastern Ukraine declared separatist republics in two cities on Monday, and Ukrainian officials accused Moscow of orchestrating the moves as the first step toward launching an invasion.

In Washington, the Obama administration expressed deep skepticism that the scattered uprisings and building takeovers in cities such as Donetsk and Kharkiv were spontaneous. “There is strong evidence suggesting some of these demonstrators were paid,” said Jay Carney, the White House press secretary.

The sudden uptick in tension, which began Sunday when pro-Russia demonstrations turned violent, has raised fears that Russia is about to try to duplicate its success in annexing Crimea last month. But the United States and other Western countries have warned that they will not stand by if that occurs.

“If Russia moves into eastern Ukraine, either overtly or covertly, this would be a very serious escalation,” Carney said.

In Donetsk, several hundred protesters who had occupied a regional administration building declared a “People’s Republic of Donetsk” and announced a referendum on secession to be held no later than May 11. They called on Russia to send in troops if they are attacked.

There was little evidence that they enjoy any public support.

The Ukrainian government dispatched its highest-level police and security officials to the region Monday in an effort to put down the separatist agitation.

Kiev is confronting an attempt to “destabilize the situation,” Prime Minister Arseniy Yatsenyuk said at an emergency cabinet meeting Monday. “The plan is for foreign troops to cross the border and seize the country’s territory, which we will not allow.”

Secretary of State John F. Kerry, in a call Monday to Russian Foreign Minister Sergei Lavrov, expressed what a spokeswoman called “great concern” about “escalatory steps” by the Russians.

Jen Psaki, the spokeswoman, said Lavrov agreed in the call to participate in talks with U.S., Ukrainian and European Union officials “in the next 10 days . . . to try and de-escalate the situation.”

In a separate conversation with Ukrainian Foreign Minister Andrii Deshchytsia, on M onday evening, Lavrov said Kiev must not use force against the pro-

Russia activists.

According to a Russian Foreign Ministry statement, Lavrov emphasized “the need to respect the aspirations of the residents of southeastern Ukraine and the inadmissibility of the use of force to respond to legal demands [by protesters] to protect their language, culture and socioeconomic rights.”

View Graphic Ukraine's ethnic russian population

Russian authorities, however, consider any protest in their country to be illegal if it hasn’t received prior approval.

Explosive situation

In Kharkiv, local reporters said a group of armed men stormed the opera house Monday thinking it was the mayor’s office. Fighting in the city continued into the evening, as armed agitators tried to break into the local security agency headquarters, but police said they were eventually turned back.

The demonstrators in the city were also demanding a referendum like the one held in Crimea after Russian troops moved into the peninsula. The situation in Kharkiv, where agitators declared a “Kharkiv National Republic,” appeared particularly combustible after pro-Ukrainian activists from Kiev reportedly headed to the city Monday.

Ukrainian news agencies reported allegations that Rinat Akhmetov, one of the country’s richest men and the overseer of a coal empire in Donetsk, is bankrolling the separatist agitators in that city. Akhmetov has long been close to former president Viktor Yanukovych, who fled to Russia in late February after mass demonstrations calling for his removal. Donetsk is Yanukovych’s home town.

The union representing the coal miners, who were once a powerful political force but have been weakened by years of cutbacks, said Monday that it does not support any move that would divide Ukraine.

For the past few weekends, pro-Russia demonstrations in Kharkiv, Donetsk and Luhansk had been orderly and diminishing in size. But Sunday, they turned dangerous as crowds broke off and began to occupy government buildings in the three cities.

In Luhansk, police said some demonstrators had entered the security services headquarters and seized guns. Police responded by setting up roadblocks around the city.

Ukrainian President Oleksandr Turchynov told the nation in a televised address Monday that Russia was trying to topple the Kiev authorities and tear the country apart.

He charged that “enemies of Ukraine are trying to repeat the Crimean scenario,” but he vowed that they would not succeed.

‘The response will be tough’

In a meeting with reporters Monday, the Ukrainian foreign minister said the interior minister, the heads of the Security Service and the National Security and Defense Council, and a deputy prime minister had gone to the eastern part of the country to bring the situation under control.

“The response will be tough,” Deshchytsia said, in contrast to what happened in Crimea. There, Russia sent in well-disciplined troops, in uniforms without insignia, who began to take over the peninsula by occupying the regional parliament building in Simferopol on Feb. 27. That day, the new government in Kiev was taking office after the ouster of Yanukovych.

Thousands of Russian troops have been camped along Ukraine’s eastern border for days, and officials in Kiev fear that Moscow has been promoting separatist sentiment and demonstrations so it can move its forces across the frontier on the pretext of restoring order and protecting a largely Russian-speaking population. Russian officials deny that they have any intention of invading Ukraine and say their troops are conducting routine exercises.

In Washington, Psaki said the movement of Russian forces into eastern Ukraine “either overtly or covertly . . . would result in additional costs” to Moscow.

The U.S. government imposed visa bans and asset freezes on high-level Russian government and business figures after Russia’s incursion into Crimea. In addition, President Obama last month signed an executive order authorizing financial sanctions against a wide range of Russian economic sectors in the event of Russian troop movement into eastern Ukraine.

In Moscow, the head of the defense and security committee of the upper house of parliament said Russia could not send peacekeepers into Donetsk, as the protesters have asked, without approval from the U.N. Security Council — which is highly unlikely to be granted.

Viktor Ozerov told the Interfax news agency that a country cannot simply send in peacekeeping troops at the request of “local authorities.” He said Crimea was an exception because Russia had military bases there under an agreement with Ukraine.

The Kiev government could not organize resistance to the Crimea takeover, which was backed by a Russian propaganda campaign that described Russian speakers in the region as under threat from fascists in Kiev. In a quickly arranged referendum March 16, Crimeans voted to join Russia, which promptly annexed the region.

In Crimea, where Ukrainian troops have been withdrawing, a Russian soldier fatally shot a Ukrainian naval officer, a spokesman for the Ukrainian military said Monday. The Ukrainian major reportedly was packing to leave the Mykolaev region when an argument broke out and the Russian fired, the spokesman said.

The new government in Kiev took over after Yanukovych fled Feb. 22. He was toppled by protesters who took to Kiev’s Independence Square, known as the Maidan, in a demonstration that began in favor of European integration and turned into a demand for good government and a fight against corruption.

Englund reported from Moscow. Karen DeYoung in Washington contributed to this report.