Kerry tells Russia of 'deep concern' over Ukraine

US Secretary of State John Kerry told Russian Foreign Minister Sergei Lavrov of his "deep concern" Tuesday over Moscow's failure to de-escalate tensions in Ukraine, a senior State Department official said.

Kerry also warned that a lack of Russian progress on a deal struck in Geneva last week would lead to more sanctions, the official added.

The latest flurry of US diplomacy came as Ukraine relaunched military operations against pro-Kremlin separatists, while Russia already has tens of thousands of its troops massed on Ukraine's eastern border.

Those moves underscored the severity of the crisis that has brought East-West relations to their most perilous point since the end of the Cold War.

In a phone call to Lavrov, Kerry "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," the senior official said.

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

The top US diplomat also spoke with Ukrainian Prime Minister Arseniy Yatsenyuk and praised the "important steps" the interim government in Kiev has taken to quell tensions.