Secretary of State John Kerry delivered a strongly worded warning to Russia on Thursday, saying the U.S. is ready to impose more sanctions if Russia refuses to de-escalate the situation in Ukraine.

"The window to change course is closing," Kerry said. If Russia doesn't change course, "the world will make sure the costs for Russia will only grow."

Kerry's statement comes after Ukraine used lethal force against pro-Moscow rebels in eastern Ukraine on Thursday. As Reuters reports, Ukrainian forces killed five separatists and in response Russia performed some military drills near the border.

Earlier today, Kerry called Russian Foreign Minister Sergei Lavrov to express his "deep concern over the lack of positive Russian steps to de-escalate the crisis."

The U.S. said it planned to deploy 600 troops to Poland and the Baltic states to "reassure allies and partners"

Of course, this all stems from Russia's annexation of Crimea. The U.S. imposed sanctions on individuals and Russian officials back in March.

Kerry said what happened in Crimea appears to be happening again in eastern Ukraine.

"Nobody should doubt Russia's hand in this," Kerry said, adding that uniformed men who have taken over official buildings in eastern Ukraine are dressed and are acting in the same manner as those who took over Crimea.

Russia, Kerry said, is behind a campaign of "distraction, deception and destabilization" in Ukraine.

In a statement, Russia's Lavrov said he told Kerry in a phone call that Ukraine is the one that needs to defuse the crisis.

Ukrainian nationalists should drop their weapons and initiate constitutional reforms, the statement said.

Russia has claimed the rebels are Ukrainian. The U.S. disputes that. Kerry called Russia's actions a "full-throated effort to actively sabotage the democratic process."

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