In a phone conversation with Ukrainian President Petro Poroshenko, Biden voiced "concern" about the militants' refusal to reciprocate Kiev's week-long ceasefire declared on Friday, Xinhua quoted a statement issued by the White House' as saying.
The statement said that Biden also "commended President Poroshenko's continued commitment to holding round-table discussions with civic leaders in eastern Ukraine, in accordance with his peace plan."
Poroshenko accused the militants of continuing to attack the Ukrainian forces, even after the ceasefire declaration.
Poroshenko accused the militants of continuing to attack the Ukrainian forces
Biden reiterated that the US was working closely with the G7 or Group of Seven industrialised nations - Britain, Canada, France, Germany, Italy and Japan- to impose further economic sanctions on Russia.
The Kremlin press service said on Saturday Russian President Vladimir Putin supported Poroshenko's order for ceasefire and the president has urged all parties to stop all hostile actions and start talks, stressing that the peace plan "will not be viable and realistic without practical steps towards negotiations."
During telephone conversations with German Chancellor Angela Merkel and French President Francois Hollande on Sunday, Putin also expressed support for Ukraine's peace settlement, the Kremlin said.
But "in this context it was stressed that the intentions announced by the Ukrainian head of state must be supported by an actual ceasefire," the Kremlin said.