WASHINGTON (AFP) - US Secretary of State John Kerry on Monday urged Russia to take "concrete steps" to implement an agreement on defusing the crisis in Ukraine.

In a phone call with Russian Foreign Minister Sergei Lavrov, Mr Kerry said the steps should include "publicly calling on separatists to vacate illegal buildings and checkpoints, accept amnesty and address their grievances politically," State Department spokesman Jen Psaki said.

"He also called on Russia to assign a senior diplomat to work with the OSCE mission in eastern Ukraine, to make absolutely clear to the separatists that Russia supports the agreement and wants de-escalation.

"He also called on Russia to speak out against the seizing of journalists and other innocents as hostages," she said.

US President Barack Obama has threatened to slap more sanctions on Moscow, beyond ones already imposed by the United States and the European Union targeting the inner circle of Russian President Vladimir Putin if the pact agreed to in Geneva on Thursday is not implemented soon.

Mr Kerry's conversation with Mr Lavrov came as US Vice President Joe Biden was beginning a two-day visit to Kiev in a show of support for its government.

The Geneva accord, signed by Ukraine, Russia, the United States and the EU, calls for all "illegal armed groups" in Ukraine to surrender their weapons and halt the occupation of public buildings and other sites.

But a brief Easter truce was broken on Sunday when two insurgents were killed in the rebel-held eastern Ukrainian city of Slavyansk.