Ukraine's acting president says its military is "on full combat alert" against a possible invasion by Russian troops massed on its border.
Acting President Oleksandr Turchynov says there is a real threat of Russia starting a war against Ukraine's mainland.
Meanwhile, pro-Russian gunmen seized another administrative building in eastern Ukraine, taking control of the city council building in Horlivka. Police say the pro-Russian rebels have also overtaken the regional police department in Horlivka.
Pro-Moscow rebels in Donetsk have set a referendum on secession for May 11. A similar vote last month led to Russia's annexation of the Crimean peninsula.
Hundreds of pro-Russian separatists overran Ukrainian government buildings near the Russian border earlier this week, seizing a prosecutor's office in Luhansk and taking control of the provincial capital Donetsk.
The latest takeovers come as negotiations resumed to gain the release of seven European observers taken hostage last week in Slovyansk.
The self-proclaimed pro-Russian mayor of that city said Tuesday he would be willing to swap the observers for pro-Russian activists held by Ukrainian authorities.
In Washington Wednesday, the top Republican on the Senate Foreign Relations committee, Bob Corker, called for deeper sanctions on Russia. He proposed sanctions on Russia's banking and energy sectors so, in his words, "we actually do something that affects the Russian economy" to the extent that they pull troops away from the Ukrainian border and remove operators inside Ukraine. He said current sanctions on Russia are inadequate.
"We as a nation so far have not signified that we are really willing to use these economic sanctions in a way, through the president's own executive order, I might add, to change behavior. And so we are very concerned about the direction this is taking."
In Moscow, Russian President Vladimir Putin has threatened to re-evaluate the role of Western investors in Russia's energy-driven economy, if sanctions imposed by Washington and the European Union continue.