SLOVYANSK, Ukraine -- Russia on Thursday began military drills on its border with Ukraine as the government there mobilized against pro-Russian militants, killing as many as five people, according to Ukrainian officials.
Russian President Vladimir Putin condemned the Ukrainian actions, and his top deputies said a Ukrainian mobilization in the restive eastern part of the country would draw a Russian response. The tit-for-tat military movements brought the two sides closer to a direct armed confrontation in a standoff that analysts call one of the most dangerous on European soil since the end of the Cold War.
"If the Kiev regime has started to use the army against the population inside the country, it beyond any doubt is a very serious crime," Mr. Putin said at a media forum in St. Petersburg. He added that if Ukrainian authorities escalated the confrontation, there would be "consequences."
After a day of increasingly dire reports from the ground, the Obama administration struck back, at least verbally. "If Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake," Secretary of State John Kerry said in Washington. Although he announced no new sanctions, Mr. Kerry said "the window [for Russia] to change course is closing. .... We are ready to act."
The pro-Russian activists were killed in fighting at three checkpoints surrounding the city of Slovyansk, and one Ukrainian soldier was wounded, the Ukrainian Interior Ministry said in a statement. The violence broke out as security forces attempted to dislodge protesters and pro-Russian militants who have encamped in cities across eastern Ukraine, with other fighting reported at an arms depot and a city hall.
The Russian military launched the "tactical drills" in the regions bordering Ukraine in response to the events, Russian Defense Minister Sergei Shoigu said at a meeting in Moscow. If Ukraine's "war machine" does not halt, it will lead to a large number of casualties, he said. "We have to react to such a development." The Russian defense minister said the drills would include aerial exercises near the border.
The leading candidate in Ukraine's upcoming presidential election, former Foreign Minister Petro Poroshenko, said three Russian military helicopters crossed the border and entered Ukrainian airspace. He said the incident was being investigated.
"Ukraine wants Russia to stop the constant threats and blackmail and to withdraw its troops from the border," Ukraine's acting president, Oleksandr Turchynov, said Thursday.
A senior U.S. official said the United States is aware of reports of helicopter incursions into Ukrainian airspace but could not confirm them. "That said, we are watching Russian troop movements on the border with great concern," the official said. "Any further Russian military intervention into Ukraine would be a grave mistake."
Mr. Kerry held Russia fully responsible for the escalating violence. "This is a full-throated effort to actively sabotage the democratic process through gross external intimidation," he said. U.S. intelligence has reported that "Russian intelligence and Special Operations" forces are "playing an active role" in eastern Ukraine. "Nobody should doubt Russia's hand in it," including with personnel, weapons, money and coordination of separatist actions, Mr. Kerry said.
Fully supporting charges levied by the Ukraine government, Mr. Kerry cited intercepted communications between known Russian agents and separatists in Ukraine as well as extensive social media photographs of Russian Special Operations members on the ground.
Noting that it has been a full week since Russia agreed in a Geneva meeting with the United States, Ukraine and Europe to help avoid further escalation, Mr. Kerry said the accord Mr. Putin's government signed "is not open to interpretation. It is not vague. It is not subjective. It is not optional."
While the Ukrainian government has kept its commitments to begin a process of constitutional reform and respect minority rights, "Russia has put its faith in distraction, deception and destabilization. ... [and] refused to take a single concrete step in the right direction," Mr. Kerry said.
President Barack Obama said Wednesday that a new round of sanctions against Russian individuals had already been drawn up, and Mr. Kerry said previous measures have already damaged Russia's first-quarter economy, including growth estimates revised downward by 2 to 3 percentage points and $70 billion in capital that has fled the Russian financial system.
In a Tokyo news conference at the first stop on his Asia trip, Mr. Obama said the United States has "teed up" additional sanctions that could be imposed against Russia unless Mr. Putin does more to encourage pro-Russian militants in eastern Ukraine to disarm. "There's always the possibility that tomorrow or the next day Russia takes another course," he said. "Do I think they're going to do that? So far, the evidence doesn't make me hopeful."