Russian Foreign Minister Sergei Lavrov told Russia Today on Wednesday that his country would respond militarily if Russia's interests came under attack. He also said Americans are directing efforts against pro-Russian activists in Ukraine. (Ruptly.TV)

Russia warned Wednesday that it was prepared to retaliate against any attack on its citizens or interests in Ukraine, as the Kiev government resumed military operations against pro-Russian militants in the eastern part of the country.

The escalation came as U.S. paratroopers landed in Poland to begin training exercises intended to demonstrate support for American allies in the region.

“If we are attacked, we would certainly respond,” Russian Foreign Minister Sergei Lavrov told the RT television network in Moscow, recalling the five-day war between Russia and Georgia in August 2008, when Moscow acted to protect pro-Russian secessionists in the breakaway Georgian region of South Ossetia.

“If our interests, our legitimate interests, the interests of Russians, have been attacked directly, like they were in South Ossetia, for example, I do not see any other way but to respond in accordance with international law,” he said.

“Russian citizens being attacked is an attack against the Russian Federation,” Lavrov said. He also accused the United States of directing the actions of the Ukrain­ian government in a “hands-on manner,” noting that Ukraine had ordered Wednesday’s military action only after a Tuesday visit from Vice President Biden.

In Washington, State Department spokeswoman Jen Psaki said it was “ludicrous” for Lavrov to claim “that the United States has anything to do with Ukraine’s counterterrorism operation or that . . . we’re running the show or funding it.” She called Lavrov’s remarks “counterproductive and inflammatory” and noted that he gave no indication of a Russian plan to implement last week’s Geneva agreement to use Moscow’s influence to disarm the separatists and push for occupied buildings to be vacated.

President Obama said last week that he hoped to see Russia follow through “over the next several days” on the agreement, reached with the United States, Ukraine and the European Union. After White House meetings this week, officials said they anticipated that U.S. asset freezes and visa bans would be announced against a new set of prominent Russians by Friday.

Lavrov’s declaration, which echoes warnings given by Russia before its annexation of Crimea last month, followed the discovery in eastern Ukraine of the body of a local politician who supported Ukrainian unity. He had been tortured and dumped in a river after being abducted last week, Ukrain­ian authorities said Wednesday.

“The terrorists who effectively took the whole Donetsk region hostage have now gone too far,” Ukraine’s acting president, Oleksandr Turchynov, said after officials identified the stabbed and bruised body of Volodymyr Rybak. His corpse and that of an unidentified man were found Saturday near Slovyansk, a city that international observers say is controlled by armed pro-Russian activists.

“These crimes are being committed with the full support and connivance of the Russian Federation,” Turchynov said.

A politician’s killing

Rybak was kidnapped last Thursday by four men in military uniforms who hustled him into a vehicle outside City Hall in Hor­livka, a depressed industrial and mining city about 26 miles northeast of the regional capital, Donetsk. Rybak, a local council member, had tried to lower the flag of the “Donetsk People’s Republic” at City Hall and replace it with the Ukrainian flag.

Rybak’s wife, Elena, said in an interview soon after her husband was taken that she feared the worst. “He was a police and knows how to defend himself,” she said. “He would not have gone anywhere without a fight.”

Her hands shook as she held a cellphone, waiting for a call. A friend told her that Rybak would call soon. “I don’t think so,” she said.

She and her adult son identified Rybak’s body Wednesday.

Vasilii Mirozhnik, who knew Rybak well at City Hall, said, “He was a regular guy, a popular guy, with strong opinions, always fighting for justice.”

Mirozhnik wore a Ukrainian flag pin on his lapel. Asked about it, he said, “Why not? It’s my country. I am proud of it.”

For many residents here, the flag they fly — Ukrainian or Russian — is of profound importance. In the past few days, people who want to remain in a united Ukraine — though with more self-government and more say over budget issues — have begun to assert themselves in rallies and flash mobs organized over social media.

Growing violence

There has been a rise in reports of beatings, disappearances and detentions in recent days. On Tuesday in Kramatorsk , the police chief was taken out of his station by armed men, the deputy mayor was beaten and the Security Service building was seized.

The Kyiv Post newspaper reported Wednesday that 12 people have been kidnapped over the past week in Slovyansk and Horlivka.

“It’s chaos,” said Yurii Zhuk, a fellow deputy on the Horlivka City Council with Rybak. He said he did not know who kidnapped and killed Rybak. “They’re outsiders,” he said. “Radical elements.”

Zhuk said that “the police were useless.” In a video taken right before Rybak was kidnapped, police officers in blue uniforms are visible in the crowd. “They’re demoralized,” Zhuk said. “They’re divided.”

Oleg Gubanov, another City Council member, said the killing of Rybak and other violence are driven by “third parties who want to destabilize the situation.”

Several Ukrainian reporters have been arrested. And an American journalist, Simon Ostrovsky, who works for Vice News, has been detained in Slovyansk since Tuesday. The leader of the pro-Russian forces in the city, Vyacheslav Ponomaryov, said Wednesday that Ostrovsky had been detained by “self-defense forces,” Interfax reported. “According to our information, he is an informer for Pravy Sektor,” the right-wing Ukrainian nationalist organization, Ponomaryov told the news agency.

“The journalist has normal living conditions and is being fed,” he said.

To the north and west of Ukraine, a company of 150 U.S. paratroopers landed in Poland on Wednesday to begin exercises with the Polish military, a move that has been greeted with relief by Eastern European leaders who fear Russian military actions near their borders. Three other similar-size companies of U.S. troops will be sent to Lithuania, Latvia and Estonia in the next several days.

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