The Sunni-ruled kingdom of Saudi Arabia is a main backer of forces fighting to topple Syria's President Bashar al-Assad, while its Shiite-dominated rival Iran backs him.
"In mid-June, Saudi leaders want to unite all -- or a vast majority -- of Syrian political and military opponents to prepare" for the period after Assad, said Haytham Manna, a veteran opposition figure.
This meeting would "not include the jihadists of (Al-Qaeda's Syria affiliate) Al-Nusra Front and of the Islamic State group," he told AFP by telephone.
Manna heads the "Cairo conference," a gathering of opposition figures living in Syria and abroad.
He said Saudi Arabia had tentatively organised its pan-opposition meeting for May 3, but was forced to abandon the idea due to differences among opposition groups.
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But closer ties between Saudi Arabia, Qatar and Turkey -- previously competing for influence over Syria's rebels -- had led to a string of recent victories for opposition fighters, said Manna.
A representative from the opposition National Coalition, which is officially recognised by much of the international community, told AFP a date was yet to be set for the conference.
"All of the political and military opposition, inside and outside Syria, would take part," the representative said.
"The objective would be to find common points among the different opposition groups and to come up with a road map for the transitional period after Assad's fall," added the official.
Saudi Arabia's foreign minister told AFP he had no information on the topic.