Defense Secretary Chuck Hagel was concerned enough about growing numbers of Russian troops near Ukraine's eastern border to have an hour-long telephone conversation with Russian Minister of Defense Sergey Shoygu about Russian intentions.

Kirby said that Hagel "was clear and he was firm" in his phone conversation with Shoygu. "Because Russian forces are in control of Crimea, they bear responsibility for what is happening there." He said Hagel also pressed Shoygu for an explanation of "Russian intentions with respect to forces they have aligned near Ukraine's eastern and southern borders."

A U.S. official told ABC News that in the past two weeks about 20,000 Russian troops have gathered along Ukraine's eastern border participating in what Shoygu characterized as exercises. The official says the Russian troops are about 75 to 100 miles from the Ukrainian border at training areas stretching along several parts of that border.

The Russian Defense Ministry had announced more than a week ago that 8,000 troops would participate in land exercises in those regions, but the official said Russia has reinforced their numbers since then.

Kirby said the increase in Russian troop numbers had raised U.S. concerns.

"I think (the call) was prompted by a number of things," Defense Department Press Secretary John Kirby said Thursday at a Pentagon briefing. "That they kept reinforcing, and that we didn't believe we had a clear indication of intent. But the call was also on the secretary's mind because of the use of force in Crimea."

Kirby said Shoygu assured Hagel "that the troops he has arrayed along the border are there to conduct exercises only, that they have no intention of crossing the border into Ukraine, and that they would take no aggressive action"

Shoygu said there was no "firm timetable" for how long the exercises would last.

Earlier in the day, President Barack Obama announced additional economic sanctions on Russian President Vladimir Putin's inner circle as a response to Russia's annexation of Crimea. These are the second set of sanctions the Obama administration has imposed on senior Russian officials in a week.

"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," Obama said. "And because of these choices, the United States is today moving, as we said we would, to impose additional costs on Russia."

Obama said he has already put plans into motion to impose sanctions on sectors of the Russian economy because of Russia positioning its military "in a way that could lead to further incursions into southern and eastern Ukraine."

The president said imposing large-scale economic sanctions is not a preferred outcome because of possible disruption of the global economy.

"However, Russia must know that further escalation will only isolate it further from the international community."