Secretary of State John Kerry called Russian Foreign Minister Sergey Lavrov on Tuesday and urged Moscow to “tone down” its rhetoric, threatening further sanctions if the Kremlin did not de-escalate the crisis in Ukraine.

“The Secretary expressed deep concern over the lack of positive Russian steps to de-escalate, cited mounting evidence that separatists continue to increase the number of buildings under occupation and take journalists and other civilians captive,” said a senior State Department official.

“He urged Russia to tone down escalatory rhetoric, engage diplomatically in the east with the OSCE [Organization for Security and Co-operation in Europe] and Ukrainian government, and issue public statements calling for those occupying buildings to disarm and stand down in exchange for amnesty,” the official added.

A diplomatic deal reached in Geneva last week called for pro-Russian separatists in eastern Ukraine to stand down and return government buildings and territory they seized in exchange for amnesty from Kiev.

But the Russian militants — who the U.S. says include members of Russia’s security forces and have direct support from Moscow — have refused to back down. Russia has also kept its troops massed on Ukraine’s border despite the agreement.

Kerry “reiterated that the absence of measurable progress on implementing the Geneva agreement will result in increased sanctions on Russia.”

The secretary also called Ukrainian Prime Minister Arseniy Yatsenyuk and “praised” the “important steps” taken by Kiev to de-escalate, the official said. Kerry encouraged Ukraine to finalize legislation which would offer amnesty to the pro-Russian groups and begin a “national dialogue” to ease tensions.

Vice President Joe Biden visited with Ukrainian leaders on Tuesday and pledged that the U.S. would offer additional aid to stabilize the country. Biden also pressed Russia to “stop talking and start acting” to implement the Geneva agreement.

But despite administration pressure, Russian officials have been slow to act, instead criticizing Ukraine's efforts.

White House press secretary Jay Carney has said that the U.S. is ready to impose further sanctions on Russia within “days” if the Kremlin does not work to defuse the crisis.

Ukraine’s acting president on Tuesday called for his country to resume security operations against the pro-Russian groups.