Eastern Ukraine is on edge as acting president Oleksandr Turchynov threatens to launch what he called a "full-scale anti-terrorist operation" against pro-Russians who have seized government buildings.
Mr. Turchynov has promised amnesty for those who give up their weapons and come out peacefully. But he says he will not let Russia take over eastern Ukraine the way it annexed Crimea last month.
So far, there are no signs the pro-Russian demonstrators are heeding the president's demands.
Russia came under heavy criticism during an emergency meeting of the U.N. Security Council Sunday night.
U.S. Ambassador Samantha Power accused the Russians of "writing and choreographing" the violence in Ukraine.
British Ambassador Mark Lyall Grant says satellite images show as many as 40,000 heavily armed Russian troops massed near the Ukrainian border.
Russia insists it has the right to protect Russian speakers it says are under threat in Ukraine. Russia accuses the interim Ukrainian government of being anti-Russian and anti-Semitic extremists.
Russian Ambassador Vitaly Churkin said the West has the power and the influence to push the Ukrainians towards dialogue and away from force.
Ukraine's Ambassador Yuriy Sergeyev said Russia's comments do not correspond with reality. He called on the Russians to immediately stop violating Ukrainian sovereignty.
Ukrainian special forces and pro-Russian militia exchanged gunfire in the eastern Ukrainian city of Slovyansk Sunday, with both sides reporting casualties. Pro-Russian gunmen had taken over the Slovyansk police station and government facilities in the cities of Donetsk and Kramatorsk.
Top diplomats from Russia, the United States, Ukraine and the European Union are set to hold emergency talks on the crisis April 17 in Geneva. White House officials say U.S. Vice President Joe Biden will travel to Kyiv April 22.