HORLIVKA, Ukraine -- 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 as a sign of support for American allies in the region.
"If we are attacked, we would certainly respond," Russian Foreign Minister Sergei Lavrov said in Moscow in an interview with the RT television network, 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. Russian citizens being attacked is an attack against the Russian Federation," Mr. Lavrov said.
The foreign minister 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 visit Tuesday from Vice President Joe Biden.
The declaration from Mr. Lavrov -- which echoes warnings the Russian government gave before Moscow's 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 kidnapped 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.
Mr. Rybak's corpse and that of another man -- so far unidentified -- 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," Mr. Turchynov said.
Mr. Rybak was kidnapped 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. Mr. 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.
Mr. Rybak's wife, Elena, said in an interview soon after her husband was taken that she feared the worst. "He was a policeman 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 and waited for a call. A friend told her that Mr. Rybak would call soon. "I don't think so," she said. She and her adult son identified Mr. Rybak's body Wednesday.
Vasilii Mirozhnik, who knew Mr. Rybak well at City Hall, said, "He was a regular guy, a popular guy, with strong opinions, always fighting for justice." Mr. 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 in Ukraine, 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 say over budget issues -- have begun to assert themselves in rallies and flash mobs organized over social media.
There has been a rise in reports of beatings, disappearances and detentions in recent days. On Tuesday, the Kramatorsk police chief was taken out of his station by armed men, the deputy mayor was beaten and the city's security service building was occupied.
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 with Mr. Rybak on the Horlivka City Council. He said he did not know who kidnapped and killed Mr. Rybak. "They're outsiders," he said. "Radical elements."
Mr. Zhuk said, "The police were useless." In a video taken right before Mr. Rybak was kidnapped, police officers in blue uniforms are visible in the crowd. "They're demoralized," he said. "They're divided."
Several Ukrainian reporters have been arrested. And an American journalist, Simon Ostrovsky, who works for Vice News, has been detained since Tuesday morning in Slovyansk. The leader of the pro-Russian forces in Slovyansk, Vyacheslav Ponomaryov, said Wednesday that Mr. Ostrovsky had been detained by "self-defense forces" in the city, Interfax reported.
To the north and west of Ukraine, a company of 150 U.S. paratroopers landed Wednesday in Poland 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.