Russian President Vladimir Putin condemned the Ukrainian government's use of force in light of clashes between Ukrainian troops and pro-Russian separatists near the city of Slovyansk on Thursday. (Video from YouTube/igordolar)

Russia on Thursday began military drills on its border with Ukraine as the government there mobilized against pro-Russian militants, killing “up to 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 elicit 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,” 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 F. Kerry said in Washington. Although he announced no new sanctions, 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 tried to dislodge protesters and pro-Russian militants encamped in cities across eastern Ukraine, with other fighting reported at an arms depot and a city hall.

The Russian military began the “tactical drills” in response to events across the border in Ukraine, Russian Defense Minister Sergei Shoigu said at a meeting in Moscow.

If Ukraine’s “war machine” does not halt, Shoigu said, it will lead to a large number of casualties. “We have to react to such a development,” he said.

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 American 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.”

The U.S. Army is sending four companies of 150 paratroopers each to Poland, Lithuania, Latvia and Estonia this week to conduct training exercises.

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, Kerry said, adding that “nobody should doubt Russia’s hand in it,” including the supply of personnel, weapons and money and the coordination of separatist actions.

Kerry, fully supporting charges levied by the Kiev government, cited intercepted communications between known Russian agents and separatists in Ukraine and extensive social media photographs of Russian special operations forces on the ground.

Noting that it has been a full week since Russia agreed in a Geneva agreement with the United States, Ukraine and the European Union to help avoid further escalation, Kerry said the accord “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 minorities’ rights, “Russia has put its faith in distraction, deception and destabilization . . . [and] refused to take a single concrete step in the right direction,” he said.

President Obama said Wednesday that a new round of sanctions against prominent Russians has been drawn up, and Kerry said previous measures have damaged Russia’s first-quarter economy, with growth estimates revised downward by 2 to 3 percentage points and $70 billion in capital fleeing the Russian financial system.

In a news conference in Tokyo on the first stop of his Asia trip, Obama warned that the United States has “teed up” additional sanctions against Russia but that Putin can still change course and encourage pro-Russian militants in eastern Ukraine to disarm.

“There’s always the possibility that tomorrow or the next day Russia takes another course,” Obama said. “Do I think they’re going to do that? So far the evidence doesn’t make me hopeful.”

‘1st step’ of anti-terror drive

In eastern Ukraine, the first signs emerged that the state was reestablishing its authority. Outside the regional capital, Donetsk, checkpoints that on Wednesday were occupied by anti-Kiev militias were being manned Thursday afternoon by members of the Ukrainian national police.

Police officers, wearing crisply pressed uniforms and carrying automatic rifles, stopped vehicles and inspected identification cards. They knocked down tents and tire barricades erected by pro-Russia militias on the highways.

There was also at least one new military checkpoint that was occupied by tanks and members of a Ukrainian airborne unit on a road outside Slovyansk. The tank gunner, who identified himself only by his first name, Vyacheslav, said the unit’s mission was to open car trunks and look for weapons.

Several journalists have been detained in Slovyansk, which international observers have said is effectively controlled by armed pro-Russian activists. American Simon Ostrovsky, a reporter for Vice News who had been held for more than 48 hours, was released late Thursday, according to Vice News.

Miroslav Rudenko, a leader of the Donetsk region’s pro-Russian “self-defense militia,” told the Moscow-based Interfax news agency that his men were attacked by Ukrainian military personnel at roadblocks near Slovyansk’s entrances. Rudenko said that at least one person died and one was injured.

“At the moment, it is very difficult to reach our militia members on their cellphones,” Rudenko told Interfax. “A ‘combat situation’ has emerged in the city.”

In Kiev, the Foreign Ministry said Ukrainian troops struck three checkpoints, killing five militiamen. During or after the fighting, the tires used to create barricades were ignited, sending black smoke into the sky.

Yuriy Lutsenko, an adviser to Ukraine’s interim president, said: “Today was the first step of the anti-terror operation. Finally, we established control on the main highway between Kharkiv and Rostov, and then we pushed the terrorists into Slovyansk.”

In Artemovsk, a town about 50 miles north of Donetsk, 70 unidentified men carrying automatic weapons and grenades attacked an arms depot, according to the Ukrainian Defense Ministry. The ministry said that Ukrainian security forces repelled the attack and that one Ukrainian soldier was wounded.

Residents blamed the attack on anti-Kiev activists seeking access to the depot.

Criminal elements blamed

Serhiy Taruta, the interim governor of the Donetsk region and one of the richest men in Ukraine, said in an interview that the militant pro-Russian separatists in cities such as Slovyansk represent only a small percentage of the local population.

“There is a local problem, and lots of criminal or half-criminal elements are engaged in looting, plundering and, unfortunately, killing,” he said. “The police have not been able to work effectively against these forces.”

Taruta said he and his team have been actively negotiating with the pro-Russian activists and anti-Kiev protesters who have taken over or surrounded public buildings across the region.

According to Interior Minister Arsen Avakov, the City Hall in Mariupol, a southeastern port city on the Sea of Azov, has been cleared of pro-Russian activists who had been occupying it for more than a week. Avakov said the mayor was back at work.

Pro-Russian protesters and masked gunmen have been occupying government buildings across eastern Ukraine for nearly two weeks and refusing to recognize the fledging Kiev government.

Avakov wrote in a Facebook post Thursday that the Mariupol City Hall “has been freed to resume work.”

But witnesses in the city said the protesters occupying the building were attacked overnight by a group of masked men armed with clubs. Five people were taken to a hospital.

The violence threatens the legitimacy of Ukrainian presidential and mayoral elections scheduled for May 25, Putin’s spokesman, Dmitry Peskov, said Thursday. His comments raised the possibility that Russia may continue refusing to recognize the Ukrainian government even after elections are held.

Russia warned Wednesday that it was prepared to retaliate against any attack on its citizens or interests in Ukraine.

Birnbaum reported from Moscow. Alex Ryabchyn in Donetsk, Will Englund in Moscow, Juliet Eilperin in Tokyo and Karen DeYoung in Washington contributed to this report.