The announcement came on the second day of a Saudi-sponsored meeting in Riyadh, where around 140 opposition figures are gathered to unify their ranks before direct talks with Syrian President Bashar al-Assad's regime on November 28.
Several rounds of talks hosted by the United Nations have failed to bring an end to the Syrian conflict, which has killed more than 330,000 people since 2011 and forced millions from their homes.
"We have agreed with the groups here in Riyadh, along with the Cairo and Moscow platforms, to form one united delegation to participate in the Geneva talks," Syrian opposition member Bassma Kodmani told reporters in Riyadh.
Kodmani added that further meetings would be held on Friday to finalise the names and number of representatives each group would have in the 50-strong unified delegation.
The delegates at the meeting have been under heavy pressure to row back on some of their more radical demands after a series of battlefield victories that have given Assad's regime the upper hand.
A Cairo-based Syrian opposition group agreed earlier Thursday to join ranks with other opposition clusters including the Saudi-backed High Negotiations Committee (HNC) -- the largest bloc in the opposition -- and the Istanbul-based National Coalition.
After prolonged discussions that dragged into the night, Moscow-based Syrian opposition group also appeared to have joined forces with that delegation.
But there were still lingering differences.
Assad fate
Kodami said the HNC and its closest allies maintained their longstanding demand that Assad step down from power as a prerequisite for a transitional phase to end the Syrian war, prompting "reservations" from the Moscow group.
Several key opposition figures boycotted the meeting, including the Moscow group's Jamil and Riad Hijab of the HNC.
Hijab stepped down as leader of the Saudi-backed HNC this week over "attempts to lower the ceiling of the revolution and prolong the regime".
Assad's fate has been a major stumbling block in multiple rounds of negotiations between the Syrian regime and the opposition.
Staffan de Mistura, the UN's Syria envoy, attended the opening session of the Riyadh gathering on Wednesday and said the goal of the meeting was to give momentum to next week's peace talks.
The Riyadh talks come as Syrian regime ally Russia is seeking to organise a "congress" to bring together Assad's forces and various opposition groups to reinvigorate the hobbled peace process.
But Kodmani remained non-committal about the opposition's participation, saying no dates or goals had been laid out for the Russian conference.
On Wednesday, Russian President Vladimir Putin hosted the leaders of Iran and Turkey for a key trilateral summit aimed at finding a political settlement of Syria's conflict. Putin also met with Assad this week.
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