The National Coalition will also boycott the Rome meeting of the mainly Western Friends of Syria group, which supports the opposition.

BBC: Syria conflict: opposition coalition pulls out of talks

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

And we're also going to continue our efforts through the "Friends of Syria" group to provide assistance to the Syrian opposition, to help them coalesce.

WHITEHOUSE: Press Briefing by Jay Carney and Ben Rhodes

We are regularly consulting with our allies and partners around the world, with members of the Security Council and the broader "Friends of Syria" group, about potential next steps.

WHITEHOUSE: Press Briefing

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

And even as we do that with Russia, we have the "Friends of Syria" group, where we are working with like-minded nations -- European partners, Arab partners, Turkey -- to put pressure on the Assad regime, to impose sanctions, and to support a transition and support the Syrian opposition.

WHITEHOUSE: Press Briefing

Mr Kerry also announced that all aid from the 11 countries known as the Friends of Syria would now be channelled through the Turkey-based Free Syrian Army group headed by Gen Salim Idris.

BBC: Syria crisis: US steps up aid to rebels at talks in Turkey

But the diplomatic wranglings, such as Annan's recent Action Group meeting in Geneva and the U.S. and Arab-backed Friends of Syria initiative, have failed to stop the killings of thousands since March 2011.

CNN: Annan: Iran has 'role to play' for Syrian peace