The US secretary of state, John Kerry, has bluntly blamed Russia of "unmistakable involvement in destabilising" Ukraine by sending "agents" to its eastern cities.
Kerry on told US Congress on Tuesday that the Kremlin was seeking to "create chaos" in Donetsk, Kharkiv and Luhansk as a pretext for more military intervention.
"Everything that we've seen in the last 48 hours, from Russian provocateurs and agents operating in eastern Ukraine, tells us that they've been sent there determined to create chaos," Kerry said, adding that he would next week meet his Russian counterpart, Sergei Lavrov, to discuss the crisis.
Shortly after Kerry's comments, Ukraine's SBU state security service said pro-Russian activists had placed explosives in a seized government office in Luhansk, and were holding about 60 people hostage.
"These actions are extremely dangerous and endanger the lives of people both inside and outside of the building. They are using terrorist measures," and SBU statement said.
Luhansk pro-Russian protesters quickly denied the claims.
"There are no explosives, no hostages. We do not need hostages to get what we want," said Anton, one of the protesters who described himself as a coordinator of the action.
In recent days pro-Russian activists seized government buildings in several cities in Ukraine's east, declaring independence and vowing to vote on splitting from Ukraine.
Ukrainian police cleared protesters from a regional administration building in Kharkiv in a lightning nighttime operation on Monday, but others held out in the eastern cities Luhansk and Donetsk.
Lavrov and Catherine Ashton, the European Union foreign policy chief, on Tuesday discussed possible international efforts to find a solution to the Ukraine crisis, the Russian foreign ministery said in a statement.
In a telephone conversation initiated by Ashton, Lavrov reaffirmed Russia's proposal for "an authentic Ukraine-wide dialogue involving all political forces and regions" aimed at reaching agreement on constitutional reforms, the statement said.
Russia has long been suggesting "federalisation" of Ukraine in order to give more autonomy to the regions of the country, claiming it would make sure ethnic Russian population were not marginalised by Kiev's central government.
Ukrainian government sees the Russian plan as an effort to break up the country.