Syria's competing opposition factions, including armed groups battling both regime forces and militants, will meet in Riyadh next week to find common ground ahead of potential negotiations with Damascus, opposition sources said Friday.
But pressure from Turkey meant the leading Kurdish political and armed groups in Syria were not invited, they said.
According to Samir Nashar, a member of the opposition National Coalition, "the meeting will be in Riyadh on Tuesday and Wednesday, and maybe Thursday if necessary."
He said the Coalition would send 20 delegates, and that 10 other prominent opposition figures had also received invitations.
Nashar said the meeting would "discuss an agreement on a transitional period" in Syria and establish a "common list" of representatives to negotiate with the regime of President Bashar al-Assad.
"This is the difficult part," Nashar told AFP.
Last month, top diplomats from 17 countries -- including key international backers and opponents of Assad -- met in Vienna in search of a political solution to Syria's war, which has seen 250,000 people killed since March 2011.
They agreed on a fixed calendar for Syria that would see a transition government set up in six months and elections within 18 months.
UN peace envoy Staffan de Mistura announced last month that Saudi Arabia would host a broad opposition conference in mid-December.
But regional rival Iran this week warned that the Saudi conference would breach declarations made by both sides in Vienna seeking a list of mutually approved opposition groups.
Iran is Assad's staunchest ally and insisted during the Vienna talks that a demand from several countries that the president be excluded from future elections be withdrawn.
Opposition figures were also quick to denounce the Vienna summit, with Nashar warning it "will not lead to a political solution".
Haytham Manna, co-founder of the opposition coalition Cairo Conference, said 20 members of his group would participate in next week's talks.
He said a total of 85 opposition figures would be present, including 15 from armed factions.
It remains unclear which armed factions will be represented at the conference, as none have publicly announced their participation.
One opposition source said the several groups invited are "not considered terrorist groups," and included the powerful Jaish al-Islam and the rebel Southern Front.
Jaish al-Islam is based near Damascus and is supported by Saudi Arabia, while the Western-backed Southern Front coalition has taken on the regime in Syria's south.
For Hassan Abdel Azim, head of the National Coordination Committee for Democratic Change, an internal opposition group, "the important thing (in Riyadh) is creating a unified delegation that will negotiate with the government."
But Nashar acknowledged that his Istanbul-based National Coalition had exerted pressure to block the powerful Kurdish Democratic Union Party (PYD) and its affiliated People's Protection Units (YPG) from being invited.
Backed by air strikes from a US-led coalition, the YPG has been the most effective fighting force against the ISIS group in north and northeast Syria.
The YPG recently allied with Arab rebel groups in northeast Syria to form the Syrian Democratic Forces, pushing ISIS out of around 200 villages there.
But the group has come under attack from Turkey, which considers it a terrorist organisation.
Nashar accused the YPG of fighting the Free Syrian Army, including rebel groups backed by the National Coalition, rather than battling the regime.
PYD head Saleh Muslim, who confirmed to AFP that he did not receive an invitation to Riyadh, rejected Nashar's claims.
"We are Syrians and we will not listen to the words of people in the hands of (Turkish President Recep Tayyip) Erdogan," he said, referring to the National Coalition.
"Given that armed groups are participating (in Riyadh), the YPG and the Syrian Democratic Forces should be present," Muslim added.