He said in a recent interview that Muslim Brotherhood members on the council were obstructing efforts to centralize financing for the opposition because they wanted to directly control the Free Syrian Army, which is currently more of a loose coalition of armed opposition groups than a unified force.

“They’re working for the future, hoping that the Tunisia and Egypt scenario repeats itself in Syria,” Mr. Hashem added, referring to postrevolt elections in which the Muslim Brotherhood offshoots in those countries emerged as the most powerful political forces.

On Friday, Mr. Hashem said he still supported arming the opposition and direct foreign military intervention. He said he understood the fears that arms could fall into the hands of Al Qaeda or other extremist groups but said that diplomacy was failing as both President Bashar al-Assad and his opposition refused to back down.

Mr. Hashem added that the rebel groups would make serious efforts to control the arms, perhaps by registering each weapon to fighters according to their Syrian identification numbers and giving arms only to those who belong to a disciplined unit.

But skeptics say that such assurances are unrealistic. Haitham al-Manna, a Paris-based leader of an opposition group with members inside and outside Syria, said that adding more weapons would be “very dangerous” and that decentralized rebel groups could not be expected to control weapons given to them.

“If what we hear is true, this is an irresponsible attitude from the Saudis,” said Mr. Manna, who opposes the use of force by the opposition and is staying away from the Istanbul meeting. He said some rebel groups were led by extremist Salafi groups, and that others had used child soldiers or foreign fighters, accusations that could not be independently verified.

The opposition has already made attempts at coordination.

In a video shown on Al Arabiya satellite television on Friday, a man identified as a Syrian Army defector with the rank of colonel said that the armed opposition had formed military councils in cities across Syria that would work with the Free Syrian Army leadership abroad.