SLOVYANSK, Ukraine — Defying warnings from Moscow not to confront pro-Russian militants entrenched in towns across eastern Ukraine, government forces Thursday revived a stalled operation to regain control by force but had little to show for their efforts other than Russian military drills on Ukraine’s border and heightened alarm about Moscow’s next move.
Russia has repeatedly denied having a hand in the unrest convulsing eastern Ukraine or any intention to invade. But an announcement Thursday by Moscow that it would immediately start military maneuvers along the border with Ukraine, and a threat by Russia’s president, Vladimir V. Putin, of unspecified consequences as a result of what he called a “serious crime,” signaled a combustible new phase in a geopolitical battle set off by the overthrow of Ukraine’s government in February.
The day’s events also buried already feeble hopes that a deal reached last Thursday in Geneva by diplomats from the European Union, Russia, Ukraine, and the United States might calm a crisis that has stirred fears of a wider conflict on control of Ukraine, a nation of 46 million that straddles a volatile fault line between Europe and Russia.
In Washington, Secretary of State John Kerry warned Russia on Thursday night that it would face additional economic sanctions if it failed to carry out that agreement. “The window to change course is closing,” he said.
Sanctions could be announced as soon as Friday if the Russians do not respond, said one senior administration official who asked not to be identified while discussing internal planning.
In his most detailed accusation of Russian interference to date, Kerry said that US intelligence services had concluded that Russia’s “military intelligence services and special operators are playing an active role in destabilizing eastern Ukraine.”
“Some of the individual special operations personnel who were active on Russia’s behalf in Chechnya, Georgia, and Crimea have been photographed in Slovyansk, Donetsk and Luhansk,” Kerry said. “Some are even bragging about it by themselves on their Russian social media sites.”
Vyachislav Ponomaryov, the de facto mayor of Slovyansk, who was installed by pro-Russian militants, said Tuesday that armed men had come to his town from outside Ukraine but insisted they were friends and volunteers, not Russian special operations forces.
On Thursday Sergei K. Shoigu, Russia’s defense minister, said drills would begin immediately involving troops in southern and western Russia, the areas surrounding Ukraine. The drills, which would also involve the air force, will include flights along the border, Shoigu said at a meeting of Russia’s top military council.
“We have to react to such developments,” he said of the Ukrainian attacks, declaring that Russia had a duty to stop “this military machine.”
However, the most violent Ukrainian operation on Thursday, against checkpoints north of Slovyansk, a small eastern city, raised fresh questions about the competence of Ukraine’s forces and the interim government’s thinking.
With armored vehicles and helicopter support, Ukrainian troops attacked crudely built checkpoints on a narrow access road. After a brief round of fighting, the forces — which the government said were a mix of regular infantry and Interior Ministry troops — withdrew, leaving rubble and burning tires behind.
The Ukrainian news media reported late Thursday that the government had decided to suspend its“antiterrorist operation” against the separatists, with The Kyiv Post quoting an official as saying this was because the risk of Russia’s invading had “grown sharply.”
The Ukrainian Foreign Ministry, according to local news media, demanded that Russia explain the purpose of its military exercises within 48 hours.
With relations between Moscow and the West at their most distrustful and tense since the height of the Cold War, Russia’s defense minister, Shoigu, cited not only the unrest in Ukraine in his announcement of military drills but also NATO’s planned exercises in Poland and the Baltic States.
Ukrainian officials teetered between declarations of determination to purge rebels from the east and alarm that Moscow might try to protect its separatist proxies in a military push.
Ukraine’s acting president, Oleksandr Turchinov, accusing Moscow of coordinating and backing “armed killer-terrorists” responsible for attacks since April 6 on government sites in at least 10 towns, said in a statement “we will not yield to the threat of terrorism and will continue to take measures to protect the life of our citizens.”
The Ukrainian authorities said that up to five pro-Russian activists died in fighting near Slovyansk. Ponomaryov, however, said that one pro-Russian separatist was killed and one wounded. He identified the dead man as Alexander V. Lubenets, and said the Ukrainian assault had involved as many as 150 troops and had been stopped in part by a minefield that separatists had laid.