The US has voiced “great concern” about heightened tension in eastern Ukraine after pro-Russia demonstrators seized government buildings in three cities.
US Secretary of State John Kerry told Russian counterpart Sergei Lavrov in a phone call that any Russian efforts at destabilisation “would incur costs”.
They discussed the possibility of direct talks within the next 10 days.
Ukraine is sending security officials to Donetsk, Luhansk and Kharkiv, after buildings there were stormed
Rebels occupying Donetsk’s regional government building declared a “people’s republic” on Monday and called for a referendum on secession from Ukraine to be held by May 11.
Russia recently annexed the Ukrainian peninsula of Crimea, where the majority of people are Russian speakers, following a referendum that Kiev and the West say was illegal.
Moscow now has thousands of troops massed along its border with eastern Ukraine. Although it insists it has no intention of invading Ukraine, it says it reserves the right to defend ethnic Russians in the country.
Russia is refusing to recognise the new authorities in Kiev who took power after pro-Moscow President Viktor Yanukovych was ousted in February.