Ukraine is struggling to keep control of key cities in its pro-Russian east as demonstrators seize government buildings and demand votes on joining Russia.
Authorities say shots were fired and at least 60 protesters arrested in Kharkiv Tuesday, while pro-Russian demonstrators held on to government offices captured Sunday in Donetsk.
Ukrainian security officials said protesters in Luhansk wired an occupied building with explosives and were holding 60 hostages.
U.S. Secretary of State John Kerry accused Russian agents and special forces of stoking separatist unrest in eastern Ukraine. He called the Russian actions "a contrived pretext for military intervention just as we saw in Crimea."
Kerry told a senate committee Tuesday that Moscow's involvement in Ukraine "clear and unmistakable." He said President Barack Obama is preparing more tough sanctions on Russia if it continues destabilizing Ukraine.
Also Tuesday, Russian Foreign Minister Sergei Lavrov said southeastern Ukraine should be included in talks about the country's future. He said Russia wants to see the largely Russian-speaking region represented in multilateral talks.
Meanwhile, U.S. Vice President Joe Biden spoke with two European leaders on the crisis in Ukraine.
Biden met with Montenegrin Prime Minister Milo Djukanovic at the White House and telephoned Slovakia's prime minister Robert Fico. Biden thanked Mr. Fico for helping Ukraine diversify its energy supplies so it is not so dependent on Russian natural gas.