The U.S. authorities continue to consider the option of imposing economic restrictive measures against Russia, White House press secretary Josh Earnest has said.

"[U.S.] President [Barack Obama] has been working very closely with our allies to ensure that we are working in close coordination to further isolate Russia, if necessary," he said at a briefing on Tuesday.

Earnest described as "more effective" the sanctions regime with respect to Russia.

He also said that Washington was "watching very closely" Moscow's role in resolving the conflict in Ukraine.