The White House has already announced the United States will suspend participation in preparatory meetings for the G-8 summit that will bring world leaders together in June in Sochi, Russia.

"Going forward, Russia's continued violation of international law will lead to greater political and economic isolation," according to a statement released by the administration.

Pressure has been mounting on Russia as leaders from the EU and the UK joined an international outcry, with EU High Representative Catherine Ashton deploring Russia's "unwarranted escalation of tensions." British Foreign Secretary William Hague was scheduled to travel to Kiev on Sunday.

During a telephone call with Putin, U.N. Secretary General Ban Ki-Moon said he told the Russian leader that it was crucial to "restore calm and proceed to an immediate de-escalation of the situation."

"Cool heads must prevail and dialogue must be the only tool in ending this crisis," he said.