President Barack Obama on Thursday announced further sanctions against prominent Russians and cleared the way for possible sanctions on key sectors of the Russian economy in response to Moscow's seizure of the Crimea region from Ukraine.

The U.S. president, speaking at the White House, said Russia's threats to southern and eastern Ukraine posed a serious risk of escalation of the crisis in the region.

"We're imposing sanctions on more senior officials of the Russian government. In addition, we are today sanctioning a number of other individuals with substantial resources and influence who provide material support to the Russian leadership, as well as a bank that provides material support to these individuals."

The White House, which on Monday announced a first round of sanctions against 11 Russians and Ukrainians it said were involved with the Crimea annexation, was expected to detail the targets of the expanded sanctions shortly after Obama spoke.

Obama said he had signed a new executive order expanding the U.S. government's authority to take measures against economic sectors.

"This is not our preferred outcome... However, Russia must know that further escalation will only isolate it further from the international community," the president said. He was speaking on the White House south lawn before leaving on a trip to Florida.

"Over the last several days, we've continued to be deeply concerned by events in Ukraine," Obama said, citing what he called an illegal referendum in Crimea, an illegitimate move to annex the territory Crimea, and "dangerous risks of escalation, including threats" to Ukraine.

"These are all choices that the Russian government has made, choices that have been rejected by the international community as well as the government of Ukraine. And because of these choices the United States is today moving, as we said we would, to impose additional costs on Russia," he said.