Under an international accord — signed in Geneva last week by Ukraine, Russia, the European Union and the United States — illegal armed groups, including the rebels occupying about a dozen public buildings in the largely Russian-speaking east, are supposed to disarm and go home.

Speaking late Thursday, U.S. Secretary of State John Kerry accused Russia of not taking "a single step" to uphold the deal and de-escalate tensions. He added that Moscow was deliberately trying to "sabotage the democratic process."

But the Kremlin, which has deployed tens of thousands of troops on Ukraine's border, insists it has the right to protect Russian speakers if they come under threat — a reason it gave for annexing the Crimean peninsula from Ukraine last month.

Russia sternly condemned Thursday's operation, with Putin the same day warning Kyiv that its continued deployment of the army against pro-Russian armed groups would constitute “a very serious crime against its own people."

Speaking at an event in St. Petersburg, Putin said any such action was "just a punitive operation, and it will of course incur consequences for the people making these decisions, including [an effect] on our interstate relations."

The operation in Slovyansk came hours after the government reported that its forces had taken control of the town hall in the city of Mariupol, and repelled an attack on an army base in the eastern town of Artemivsk, the ministries said.

Pro-Russian rebel sources also reported the loss of the town hall in Mariupol. The city was the scene of an attack on Ukrainian troops last week that left three rebels dead, though they continued to hold the town hall, as they had since April 13.

"The town hall is liberated and can function normally," Arsen Avakov, Ukraine's interior minister, said on his Facebook page.

The Defense Ministry said in a statement that Ukrainian forces also repelled nearly 100 pro-Russian rebels in an attack on the military base in Artemivsk, just north of rebel-held Donetsk.

"The attackers were repelled and suffered significant losses," said Oleksandr Turchynov, Ukraine's acting president.