A day after an international deal in Geneva to defuse the East-West crisis in Ukraine, pro-Russian activists vowed not to end their occupation of public buildings and Washington threatened further sanctions on Moscow if the stalemate continued, according to Reuters.

Leaders of gunmen who have taken over city halls and other sites in and around Donetsk this month in pursuit of demands for a Crimea-style referendum on union with Russia rejected the agreement struck in Geneva by Ukraine, Russia, the United States and European Union and demanded on Friday, April 18 that the leaders of the Kyiv uprising must first quit their own government offices.

Moscow renewed its insistence that it has no control over the "little green men" who, as before Russia annexed Crimea last month, appeared in combat gear and with automatic weapons to seize public buildings - a denial that Western allies of those who overthrew the pro-Russian president in Kyiv do not accept.

The White House renewed President Barack Obama's demands that the Kremlin use what Washington believes is its influence over the activists to get them to vacate the premises. It warned of heavier economic sanctions than those already imposed over Crimea if Moscow failed to uphold the Geneva deal - or if it moved to send troops massed on the border into Ukraine.

"We believe that Russia has considerable influence over the actions of those who have been engaged in destabilizing activities in eastern Ukraine," national security adviser Susan Rice said. "If we don't see action commensurate with the commitments that Russia has made yesterday in Geneva ... then obviously we've been very clear that we and our European partners remain ready to impose additional costs on Russia.

"Those costs and sanctions could include targeting very significant sectors of the Russian economy."

President Vladimir Putin's spokesman hit back, while voicing skepticism - of a kind also heard from the Ukrainian government - about how useful the cautiously worded Geneva pact would be.

"You can't treat Russia like a guilty schoolboy," said Dmitry Peskov. "That kind of language is unacceptable."

The Russian Foreign Ministry said: "The Americans are once again stubbornly trying to whitewash the actions of the Kyiv authorities, who have embarked on a course of violently suppressing protesters in the southeast who are expressing their legitimate indignation over the infringements of their rights."

Later in the day, U.S. Secretary of State John Kerry called Russian Foreign Minister Sergei Lavrov and urged "full and immediate compliance" with the Geneva agreement, a senior State Department official said.

"He made clear that the next few days would be a pivotal period for all sides to implement the statement's provisions, particularly that all illegal armed groups must be disarmed and all illegally seized buildings must be returned to legitimate owners," the official said.