Syria's fragmented opposition announced early Friday they had reached an agreement to send a united delegation to next week's UN-brokered peace talks.

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 in Geneva on November 28.

Several rounds of talks hosted by the United Nations have failed to bring an end to the six-year war in Syria, which has killed more than 330,000 people since 2011 and forced millions from their homes.

"We have agreed with the two other (opposition) branches to send a united delegation to participate in the direct negotiations in Geneva," Bassma Kodmani, a member of the Syrian opposition's High Negotiations Committee, 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 was reported to have 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.

Kodmani's announcement implies that a Moscow-based Syrian opposition group has also agreed to join forces with that delegation.

Several key opposition figures have 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.

The HNC and its closest allies have consistently demanded Assad step down from power as a prerequisite for a transitional phase to end the Syrian war.

Other branches of the opposition have taken a softer stance against Assad.

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 peace process.

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.