A masked pro-Russian activist stands at a barricade outside the regional police building seized by separatists in the eastern Ukrainian town of Sloviansk on Monday. US Vice-President Joe Biden was to begin a two-day visit to Ukraine amid Russian "outrage" over a deadly weekend shootout in the rebel east that shattered a fragile Easter truce. Kirill Kudryavtsev / Agence France-Presse

Russia accused Ukraine on Monday of flouting an international accord meant to diffuse the crisis over its separatist east, as US Vice-President Joe Biden was to about to arrive in Kiev in a show of support for its pro-Western leaders.

"The Geneva accord is not only not being fulfilled, but steps are being taken, primarily by those who seized power in Kiev, that are grossly breaching the agreements reached," Russian Foreign Minister Sergei Lavrov said at a Moscow news conference.

He was retaliating to charges from Washington that Moscow is dragging its feet on implementation of the accord hammered out on Thursday in Geneva between Russia, Ukraine, the United States and the European Union.

The pact has been badly undermined by a deadly shootout in Ukraine's restive east on Sunday, and an obstinate refusal to stand down by pro-Kremlin militants who have seized control of nearly a dozen towns in the region.

The accord calls for all "illegal armed groups" in Ukraine to surrender their weapons and halt the occupation of public buildings and other sites.

Washington has warned Moscow - which it believes is pulling the strings in Ukraine's insurgency - that time is running out for the accord to be put into practice.

But Moscow in turn has cautioned that it will not tolerate further US sanctions if the deal falls apart, while stressing that it has tens of thousands of troops massed on Ukraine's doorstep.

It says Ukraine's leaders - whom it sees as illegitimate - are using force against the separatist "protesters". It also wants to see anti-Russian demonstrators in Kiev cease their occupation of the capital's main square in line with the Geneva agreement.

Lavrov said that efforts to cut Moscow off from the international community through sanctions would prove fruitless.

"Attempts to isolate Russia have absolutely no future because isolating Russia from the rest of the world is impossible," he said.

The crisis is the worst buffeting of East-West relations since the Cold War.

Biden was expected to reassure Ukrainian leaders of continued US support during his two-day visit to Kiev.

The White House said he would "consult on the latest developments in east Ukraine" during his trip.

On Monday, Biden was due to speak with US embassy officials in the Ukrainian capital. On Tuesday he was scheduled to meet with the country's interim president, Oleksandr Turchynov, Prime Minister Arseniy Yatsenyuk and lawmakers.

The US and its NATO allies have bolstered military deployments in Eastern Europe. Washington and Brussels have also pledged billions to shore up Ukraine's battered economy.

'Virtual war'

In Ukraine's east, the situation appeared calm on Monday, with insurgents still firmly entrenched in public buildings they have occupied for over a week.

"There was no shooting overnight," said Yevgen Gorbik, a rebel wearing camouflage and a military cap and standing at a barricade in the flashpoint town of Sloviansk.

"We will only shoot if attacked," he added.

Gorbik summed up the bellicose posturing and political jockeying by saying, "Currently, we have a virtual president in Ukraine, a virtual army and a virtual war".

AFP-Reuters