Syria's fragmented opposition doubled down on attempts to unify their ranks on Thursday, the second day of a Saudi-sponsored meeting ahead of next week's UN-brokered peace talks.

Around 140 opposition figures are gathered in Riyadh in a bid to form a unified delegation to meet representatives of 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.

A news conference scheduled for Thursday afternoon in Riyadh was postponed as the meeting stretched into the late evening in a bid to find consensus, according to opposition members.

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 initially reported to have agreed to join ranks with other opposition clusters including the Saudi-backed High Negotiations Committee (HNC) -- the largest block in the opposition -- and the Istanbul-based National Coalition.

"The Cairo group has joined the main opposition delegation to Geneva," Ahmed Ramadan, head of the National Coalition's media department, told AFP.

But some delegates later disputed that claim.

A Moscow-based Syrian opposition group has yet to announce if it will join forces with that delegation.

"We are looking to form one delegation with fair representation. This is the point that is currently under discussion," said Qadri Jamil, head of the Moscow group and a former Syrian deputy prime minister.

- Disputes -

Some delegates said there were lingering differences over the number of representatives each group would have in a unified delegation.

AFP received a preliminary list of 50 representatives who would be part of the delegation, but there was no formal confirmation from the delegates.

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.