U.S. President Barack Obama's national security adviser, Susan Rice, has warned Russia against sending military forces into Ukraine. 

Speaking on U.S. television, she said it would be a "grave mistake" and not in the interests of Russia, Europe or the U.S. to see Ukraine split apart.

Meanwhile, U.S. Treasury Secretary, Jack Lew, says the United States is ready to work with Russia and other countries with what he called "restoring democracy" in Ukraine. He made the comments after talks with his Russian counterpart, Anton Siluanov, on the sidelines of a G-20 meeting in Sydney.

"I had a very good conversation with my Russian counterpart. I thought there was broad agreement that the best approach for Ukraine is to get its economic reforms in place and to take advantage of the opportunity to use the combination of international support through the IMF and bi-lateral support but ultimately Ukraine is going to need to have that kind of economic transition while its going through political transition," Lew said.