The Syrian opposition leaders met at a luxury hotel in Doha, the capital of the small Gulf state of Qatar that has emerged as a major backer of the Syrian rebels. Organizers said more than 400 delegates are attending four days of internal SNC meetings and will choose new SNC leaders on Tuesday. A day later, the SNC is to vote on Seif's plan.
On Thursday, Seif will attempt to form the new leadership group with the backing of the SNC. If he is successful, the Friends of Syria, an alliance of countries backing the rebels, is to convene in Morocco, he said.
The 66-year-old Seif, who left Syria few months ago after having been detained by the regime, said that if his plan is accepted, "the whole world will be behind" the new opposition leadership.
At the Morocco conference, "maybe 100 countries will recognize this new leadership as the legitimate and only representative of the Syrians," said Seif, who suffers from cancer and is not seeking a leadership role.
He did not say what kind of practical support the opposition could expect, but suggested the Morocco gathering would be a launching pad for a transitional government.
A senior U.S. official has said the Washington did not want to attend another Friends of Syria conference unless the opposition comes up with a new, more representative leadership. Many of the current SNC leaders live in exile, and appear to have little say over the course of the rebellion inside the country.
Sieda bristled as the criticism, saying that "it is unfair to say that the SNC represents (those) outside Syria."
The SNC argues that it represents several dozen groups, including a number based in Syria.
Abdel-Rahman al-Haj, a spokesman for the group, said the international community's criticism of the SNC is meant to deflect from the world's failure to stop the bloodshed in Syria.
"The international community ... did not give us any help, support and weapons," he said. "We just hear talking."
Al-Haj, a 41-year-old Syrian exile in Malaysia, said the SNC is considering the possibility of setting up its own transitional government if it decides to reject Seif's plan. However, he said everything is still open to debate.
The disagreements at the conference reinforced doubts in the opposition's ability create a new structure the U.S. and its allies can work with. The U.S. hopes a more representative body can provide a reliable partner, buffer against interference by extremists and help bring Syria's allies Russia and China on board with change.
However, divisions among political leaders are not the only concern.
Rebel fighters are split into small largely autonomous groups, some led by local figures little known outside Syria. Most nominally belong to the umbrella Free Syrian Army, but their ties to it are often just lip-service. In many hotspots, fighters from a radical Islamic group inspired by the al-Qaida terror network have taken on prominent roles.
In his opening speech at the conference, Sieda said the SNC is trying to unify all military groups under one leadership, in part to prevent "any extremism, mistakes or atrocities" from being committed. Earlier this week, a video appeared to show rebel fighters killing a group of unarmed, captured Syrian soldiers execution-style, prompting an international outcry.
Sieda urged all commanders and rebel fighters to respect human rights and said those violating them on the rebel side would be brought to justice.
Associated Press writers Abdullah Rebhy in Doha, Qatar, and Aya Batrawy in Cairo contributed.