Pro-Russian demonstrators occupying state buildings in eastern Ukrainian cities have defied the deadline set by Kiev's interim administration to lay down their arms, even as Vladimir Putin and Barack Obama held a telephone conversation to resolve the crisis.
The anti-Kiev protesters continued to storm government buildings in the eastern region, stepping up their armed campaign against Ukraine's demands for a referendum.
Meanwhile, President Obama accused Russia of orchestrating the latest events in the Ukrainian crisis and insisted Moscow's actions are not conducive to a diplomatic solution.
The White House said the conversation between Obama and Putin was frank, adding: "The [US] president emphasised that all irregular forces in the country need to lay down their arms, and he urged President Putin to use his influence with these armed, pro-Russian groups to convince them to depart the buildings they have seized."
For his part, Putin urged Obama to use his influence in Ukraine to prevent more bloodshed.
A Kremlin statement said: "The Russian side stressed that the protests in Donetsk, Lugansk, Kharkov, Slavyansk and other cities in southeastern Ukraine are the result of the Kiev authorities' unwillingness and inability to take into account the interests of the Russian and Russian-speaking population.
"Vladimir Putin called upon Barack Obama to use the American side's capabilities to prevent the use of force and bloodshed."
Western powers have blamed Russia for the unrest, and as part of their strategy to pile more pressure on Moscow, the European Union along with the US are planning to widen the economic sanctions on Russian interests.
EU foreign ministers said they would be targeting more Russian individuals.