If Russia continues to stoke unrest in Ukraine, Russia will face further sanctions, Secretary of State John Kerry said Thursday afternoon.
"It will not just be a grave mistake, it will be an expensive mistake," he said. Kerry accused Russia of acting deceitfully and purposefully destabilizing the region. "Nobody should doubt Russia's hand in this," he said.
"Let's get real. The Geneva agreement is not open to interpretation," he said, noting that "Only one side is keeping its word."
Kerry's comments come one week after global leaders met in Geneva and agreed to de-escalate the situation in Ukraine.
"Ukraine is taking concrete steps to move past" the recent weeks of unrest, he said, noting that no Russian officials have publicly pushed for upholding the Geneva terms of diplomatic resolution. They have, instead, steadily stoked unrest.
Ukrainian forces killed up to five pro-Moscow rebels on Thursday as they closed in on the separatists' military stronghold in the east and Russia launched army drills near the border in response, raising fears its troops would go in.
Under an international accord signed in Geneva last week, illegal armed groups in Ukraine, including the rebels occupying about a dozen public buildings in the largely Russian-speaking east, are supposed to disarm and go home.
Read MoreRussia is already feeling sanctions pain
So far, the United States and EU have imposed visa bans and asset freezes on a few Russians in protest at Moscow's annexation last month of Crimea from Ukraine.
In NATO member Poland, the first group of a contingent of around 600 U.S. soldiers arrived on Wednesday. They are part of an effort by Washington to reassure eastern European allies who are worried by the Russian build-up near Ukraine's borders.
—CNBC.com with Reuters