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 Kyiv 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 manoeuvrs 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 Kyiv 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 "co-ordinating 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 manoeuvrs 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.
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.