US officials were on Thursday warning Russia over the Ukrainian crisis, saying Moscow made “expensive mistake” and will pay a “big price” for that.

US officials were on Thursday warning Russia over the Ukrainian crisis, saying Moscow made “expensive mistake” and will pay a “big price” for that.

US Secretary of State John Kerry warned that Russia's refusal to take any steps to end the crisis in Ukraine would prove costly, saying the window for Moscow to change course was closing.

He accused Moscow of a "full-throated effort to actively sabotage the democratic process through gross external intimidation" and described new Russian military exercises on the border of Ukraine on Thursday as "threatening".

"Let me be clear: if Russia continues in this direction it will not just be a grave mistake, it will be an expensive mistake," the veteran diplomat said, adding "we are ready to act" as Washington tees up new economic sanctions against Moscow.

For its part, Former US Secretary of State Hillary Clinton has called on President Barack Obama to ratchet up sanctions against Russia over Crimea and eastern Ukraine.

Speaking at a University of Connecticut contemporary issues forum, Clinton said that the sanctions against Moscow must be "tightened and widened" to prevent the Ukraine crisis from escalating.

"I think Russia will pay a big price for this," Clinton said. "But that is an endpoint that we've got to get to as peacefully as possible without seeing the total disintegration of Ukraine as a country with territorial integrity and opportunity to have the relationship it wants with the West."