SLOVYANSK, Ukraine -- Russia announced new military exercises Thursday involving ground and air forces near its border with Ukraine, swiftly responding to a Ukrainian operation to drive pro-Russia insurgents out of occupied buildings in the country's tumultuous east.
The Ukrainian move, which killed at least two people, brought new threats from Russian President Vladimir Putin, who denounced it as a "punitive operation."
"If the Kiev government is using the army against its own people, this is clearly a grave crime," Putin said.
Putin's statement and the announcement of new military maneuvers sharpened anxiety over the prospect of a Russian incursion into Ukraine. Russia's foreign minister warned a day earlier that any attack on Russian citizens or interests in eastern Ukraine would bring a strong response.
Secretary of State John Kerry quickly denounced the Russian actions, and in unusually blunt language warned that unless Moscow took immediate steps to de-escalate the situation, Washington would have no choice but to impose additional sanctions.
"Following today's threatening movement of Russian troops right up to Ukraine's border, let me be clear: If Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake," Kerry said. "The window to change course is closing,"
Accusing Russia of fomenting unrest and separatist sentiment in eastern Ukraine following its annexation of the strategic Crimean Peninsula, Kerry added: "Nobody should doubt Russia's hand in this."
"What is happening in eastern Ukraine is a military operation that is well-planned and organized, and we assess that it is being carried out at the direction of Russia," the U.S. secretary of state said.
Animosity between Moscow and Kiev has been high since the ouster of Russia-friendly president Viktor Yanukovych in February in the wake of months of protests. Russia contends the government that took over consists of nationalists who aim to suppress the large Russian-speaking population in Ukraine's east.
In March, Russia annexed the Crimean Peninsula after its residents voted to split off from Ukraine. Russian troops backed up local militias that blocked off Ukrainian military bases in the run-up to the referendum.
Ukraine's acting president accused Russia of backing the separatists in the east and demanded that Moscow stop its intimidation campaign, and leave his country alone.
Oleksandr Turchynov said in an address to the nation Thursday that Russia was "coordinating and openly supporting terrorist killers" in eastern Ukraine, where government buildings in at least 10 cities have been seized by pro-Russia gunmen.
Turchynov said Russia must pull back its troops from the Ukrainian border and "stop the constant threats and blackmail."
His foreign minister, on a visit to Prague, also blasted the Russian decision to start new military maneuvers and said his country would fight any invading troops.
"We will now fight with Russian troops if ... they invade Ukraine. Ukrainian people and the Ukrainian army are ready to do this," Andriy Deshchytisa told The Associated Press.
Russia already has tens of thousands of troops stationed in regions along its border with Ukraine. The latest Russian military exercises involve ground troops in the south and the west and the air forces patrolling the border, Russian Defense Minister Sergei Shoigu said.
Ukraine and Russia reached a deal in Geneva last week to defuse the crisis, but pro-Russian insurgents in the east -- and nationalist militants in Kiev -- have defied calls for all sides to disarm and to vacate the buildings they are occupying.
NATO Deputy Secretary General Alexander Vershbow sharply criticized Russia for making "veiled threats" and said Russia should pull its troops back to their barracks.
The Ukrainian government and the West worry that Putin would welcome a pretext for a military intervention in eastern Ukraine. Putin denies that any Russian agents are operating there, but insists he has the right to intervene to protect the ethnic Russians who make up a sizeable minority in the east.
Earlier in Tokyo, President Barack Obama accused Moscow of failing to live up to "the spirit or the letter" of last week's deal to ease tensions in Ukraine. If that continues, Obama said, "there will be further consequences and we will ramp up further sanctions."
With no appetite in the U.S. for a military response, Obama is largely banking on Putin caving under a cascade of economic sanctions targeting his closest associates. But the success of that strategy also depends on European nations with closer financial ties to Moscow taking similar action, despite their concerns about a boomerang effect on their own economies.
"I understand that additional sanctions may not change Mr. Putin's calculus," Obama said. "How well they change his calculus in part depends on not only us applying sanctions, but also the cooperation of other countries."
Meanwhile, an American journalist who had been held by insurgents in the eastern city of Slovyansk was freed Thursday.
Simon Ostrovsky of Vice News told the AP in a brief telephone call that he had been freed and was heading to Donetsk, the largest city in the region. He did not give details of his seizure or his release.
Ostrovsky went missing early Tuesday in Slovyansk. A spokeswoman for the insurgents later said he was being held at a local Ukrainian security service that had been seized earlier.
Slovyansk, located 100 miles (160 kilometers) west of the Russian border, has emerged as the focus of the armed insurgency.
The Ukrainian Interior Ministry said military and special police forces killed "up to five terrorists" while destroying three checkpoints north of Slovyansk on Thursday. One government security service member was wounded, it said.
Stella Khorosheva, a spokeswoman for the Slovyansk insurgents, said two pro-Russia fighters were killed at a checkpoint in the village of Khrestyshche, six miles north of the city. She said checks were being made at hospitals to see if there were other casualties.
The situation was quiet in Slovyansk itself, but checkpoints inside the city were abandoned and it was unclear where the pro-Russia insurgents manning them had gone.
Khorosheva said the pro-Russia militia later regained control over the checkpoints where the clashes took place. By Thursday afternoon, an AP reporter confirmed that some of those checkpoints were back in the hands of insurgents.
Khorosheva declared that the fighters were ready to repel any attack by government troops.
"We will defend ourselves to our last drop of blood. We are ready to repeat Stalingrad," she told the AP, invoking the memory of the Soviet army's victory over German forces in 1942-43.
At least 10 Ukrainian government armored vehicles were seen on the road north of Slovyansk and two helicopters circled over the area. Troops ordered residents to keep away during the operation.
Near the town of Makatikha, several miles north of Slovyansk, pro-Russia militia set fire to rows of car tires in an apparent attempt to reduce the visibility from the air. An AP reporter saw about two dozen militiamen manning checkpoints along the road earlier in the day.
Interior Minister Arsen Avakov said his forces had cleared city hall in Mariupol of the pro-Russia protesters who had been occupying it for more than a week. He provided no details of the operation in the city, which sits along the main road between mainland Russia and Crimea.
Yulia Lasazan, a spokeswoman for Mariupol's police department, told the AP about 30 masked men armed with baseball bats stormed the building before dawn Thursday and started beating the pro-Russia protesters. Five people were taken to a hospital, she said.