Moscow and Washington each called for more action from the other to save a troubled deal aimed at easing the crisis between Ukraine's new Western-backed government and pro-Russian separatists in the east.

As US Vice President Joe Biden began a two-day visit to Kiev on Monday, in a pointed show of American backing for Ukraine's new leaders, Washington and Moscow put a radically different spin on a telephone call between their diplomatic chiefs on reviving the accord reached last week in Geneva.

Under the deal, signed by Ukraine, Russia, the United States and the European Union, pro-Kremlin rebels holding a string of eastern towns were supposed to disarm and give up the buildings they have seized.

US Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov on Monday each urged the other to put pressure on his side in the crisis.

Moscow accused the Ukrainian government of "grossly breaching" the Geneva deal; in Lavrov's, on the separatist rebels, which Washington sees as backed by Russia.

Kerry told Lavrov that "concrete steps" to defuse the crisis should include "publicly calling on separatists to vacate illegal buildings and checkpoints, accept amnesty and address their grievances politically", said State Department spokeswoman Jen Psaki.

In Moscow, the Russian foreign ministry said Lavrov had asked Kerry to "pressure Kiev to stop hotheads from provoking a bloody conflict and to encourage the Ukrainian authorities to strictly fulfil their obligations", the Russian foreign ministry said.

Lavrov also accused Ukraine's government of an "inability and unwillingness" to rein in Pravy Sektor ["Right Sector"], an ultra-nationalist group the separatists blamed for an attack Sunday on one of their checkpoints in the flashpoint town of Slavyansk.

Images of 'Russian forces'

It comes as the US State Department released images on Monday of soldiers in eastern Ukraine that it says are Russian forces, showing people wearing similar uniforms and brandishing Russian weapons.

The pictures, released on Monday, mark the latest effort by the West to prove that Moscow is using its military to stir unrest in Ukraine.

A copy of the 11-page State Department document was given to reporters. The images cannot be independently verified.

It includes pictures captioned as "concerned citizens" in Crimea and the city of Slovyansk who are dressed the same and carrying the same kind of RPG-30 rocket launchers that are issued to Russian troops.

There was no way to immediately verify the photographs, which were either taken from the Internet or were given to the Organization for Security and Cooperation in Europe last week by Ukraine diplomats.

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.