Syria's main anti-regime group on Friday accused Washington of undermining the country's revolution by seeking an opposition overhaul, as rebel forces made important gains in the country's northwest.

Criticism mounted meanwhile of a video purported to show rebel fighters executing Syrian soldiers, with the UN human rights body saying it appeared to show a war crime.

Two days ahead of key talks among the opposition in Qatar, the Syrian National Council (SNC) lashed out at US accusations that it was not fully representative of the country's diverse dissident forces.

"Any discussions aimed at passing over the SNC or at creating new bodies to replace it are an attempt to undermine the Syrian revolution by sowing the seeds of division," the SNC said.

US Secretary of State Hillary Clinton this week voiced frustration with the SNC, saying it was not representative of on-the-ground opposition forces and that it "can no longer be viewed as the visible leader of the opposition."

Reports have emerged that Washington will be pushing for an overhaul of the opposition at Qatar meeting.