SLOVYANSK, Ukraine--Ukrainian security forces killed “up to five” pro-Russian activists Thursday in the restive eastern part of the country, the Ukrainian Interior Ministry said, as Russian President Vladimir Putin condemned any use of the Ukrainian military against its own citizens.

The pro-Russian activists were killed in fighting at three checkpoints surrounding the city of Slovyansk, and one Ukrainian soldier was wounded, the Interior Ministry said in a statement. The violence broke out as clashes involving Ukrainian security forces, local protesters and pro-Russian militants spread across eastern Ukraine, with other fighting reported at an arms depot and a city hall.

The Russian military launched “tactical drills” Thursday in the regions bordering Ukraine in response to events across the frontier, 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 event was being investigated. No Russian troops have crossed the border, he said.

“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.

Putin spoke forcefully against the Ukrainian military action as the clashes were underway.

“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.

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

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

There was also at least one new military checkpoint 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.

In a news conference in Tokyo on the first stop of an Asian trip, President Obama said the United States has “teed up” additional sanctions that could be imposed against Russia unless Putin does more to encourage pro-Russian militants in eastern Ukraine to put down their arms.

“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.”

In Slovyansk, which international observers have said is effectively controlled by armed pro-Russian activists, a Moscow supporter who calls himself the “people’s mayor” threatened to protect his men with hostages.

Several journalists have been detained in Slovyansk, including Simon Ostrovsky, an American reporter for Vice News, who has been held for more than 48 hours. His last tweet referred to the people’s mayor.

“Now he’s not letting reporters leave the press conference: ‘you’ll go as you came in. In a group.’ That’s one way to guarantee coverage,” Ostrovsky wrote on Tuesday.

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 confirmed at least one dead and one 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 the barricades were ignited, and black smoke poured 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 the we pushed the terrorists into Slovyansk.”

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

Local residents said they believed the attack came from anti-Kiev activists seeking access to the weapons depot.

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 Ukrainian port city on the Sea of Azov, has been cleared of pro-Russian protesters 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 Ukraine’s fledging government.

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

But eyewitnesses in the city said the anti-Kiev protesters who were occupying the building were first attacked overnight by a group of masked men armed with clubs. Five people were taken to a hospital for treatment of injuries.

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

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 Ukrain¬ian 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. (Source: The Washington Post)

Story Date: April 25, 2014