Ukrainian authorities say they have retaken the main government building from pro-Russian protesters in the eastern city of Kharkiv.
They say 70 people were arrested in an anti-terrorist operation.
Russia meanwhile has said Ukraine should avoid military preparations that could spark a civil war.
On Monday, Washington warned Moscow against moving “overtly or covertly” into eastern Ukraine, saying there’s evidence that pro-Russian protesters are being paid.
Separatists are holding out in two other Ukrainian cities, Donetsk and Luhansk, in what Kyiv has called a Russian-led plan to dismember the country.
In Donetsk, the Russian flag still flew above the regional government building on Tuesday morning.
Rebels have called for a referendum on secession from Ukraine.
One pro-Russian protester said:
“We hope people watching now will be loyal, because we will not leave this place until we make a referendum happen.”
Another man had a different vision: “I hope that we will have a united Ukraine, but as a federal state,” he said.
Reports quoting Ukrainian authorities say they have promised not to storm the Donetsk government building.
After overnight talks, protesters reportedly gave up some weapons, and agreed to withdraw their earlier declaration of a “people’s republic”.