Susan Rice, U.S. ambassador to the United Nations, told CNN's "The Situation Room" that the United States will push other approaches to the conflict, including through the Friends of Syria Group.

CNN: Syria shows al-Assad, amid speculation on his whereabouts

Meanwhile, the US State Department said that efforts were continuing to put together a "friends of Syria" group of countries.

BBC: Syria crisis: Obama condemns onslaught on Homs

Russia - a close ally of President Assad - has said the promises of aid to the opposition made by the US and other countries in the "Friends of Syria" group will encourage further violence rather than a negotiated solution.

BBC: John Kerry holds talks on Syria crisis in Ankara