White House press secretary Jay Carney called a Russian fighter jet's buzzing of a U.S. Navy destroyer in the Black Sea one of a series of Russian “provocations” and threatened that there would be costs for Moscow's aggressive actions in Ukraine.

“I can assure you that Russia's provocations and further transgressions will come with a cost,” he said. “Certainly if they go further down the road in attempting to destabilize Ukraine ... the costs will continue to grow.”

U.S. officials are actively evaluating the escalating situation in eastern Ukraine, where heavily armed pro-Russian forces have seized buildings in several cities and are not responding to Kiev's demands for them to leave.

Carney said the U.S. will consider next steps including a new round of sanctions against Russia in concert with European partners. European foreign ministers are meeting in Luxembourg Monday to weigh new sanctions against Russian officials because of Moscow's annexation of Crimea.

He also confirmed reports that CIA Director John Brennan was in Kiev this weekend as part of a broader trip to Europe. He said he was there to meet with his counterparts in the region.

Obama will call Russian President Vladimir Putin "very soon," possibly within hours, Carney said. He has already talked to French President Francois Hollande, who is pushing for greater sanctions against Russia.

While Carney insisted the U.S. is not re-engaged in a Cold War with Russia despite the considerable tensions between the two world powers over Ukraine.

“We have profound differences with Russia, and we confront those differences directly,” Carney said.

He said the Russian military had taken “provocative and unprofessional” actions, including buzzing the U.S. navy ship in the region but stopped short of saying the U.S. would impose sanctions or take action unilaterally.

With Russian troops amassed at Ukraine's eastern border poised to invade at any moment, Carney repeatedly praised the government in Kiev for showing restraint in not firing any shots. Ukraine's acting President Oleksandr Turchynov also has said he is open to the possibility of a referendum to provide greater autonomy to the country's pro-Russian regions.

This story was first published at 1:37 p.m. and has been updated.