A Syrian opposition meeting began in Riyadh on Wednesday in a bid to unify the group's position ahead of peace talks backed by the United Nations to end the country's six-year civil war.
Saudi Arabia backs the High Negotiations Committee (HNC) group whose leader, former Syrian prime minister Riyad Hijab, resigned on Monday without explanation.
The summit comes after a surprise visit by Syrian President Bashar al-Assad to Russia a day earlier to meet President Vladimir Putin, who later discussed the conflict with U.S. President Donald Trump and other Middle East leaders.
Assad has made major gains against opposition forces and Islamic State militants with the help of Russia as well as Iran, the arch-rival of Saudi Arabia, which backs Syrian rebels and had long maintained that Assad should have no role in any transition to bring the war there to an end.
U.N. peace talks mediator Staffan de Mistura urged the opposition figures gathered at a five-star hotel in Riyadh to have the "hard discussions" necessary to reach a "common line".
"A strong unified team is a creative partner in Geneva and we need that, one who can actually explore more than one way to arrive to the goals that we need to have," he said in opening remarks.
It was not immediately clear how Hijab's absence would affect the talks. Several opposition figures said he had taken an uncompromising line that rejected a role for Assad in a U.N. sponsored Geneva based peace process.
Some opposition members have hinted that the new communique would drop the long standing demand by the Riyadh based main opposition, referring to the next round of U.N.-sponsored talks.
The summit, which Saudi Arabia called "expanded", was opened to more than 140 opposition figures from the Turkey-based coalition and mainstream Free Syrian Army factions as well as independents including about a dozen women.
The HNC has represented the Syrian opposition at previous Geneva talks, while a number of other political opposition groups and figures backed by other countries including Russia and Egypt also exist.
Saudi Foreign Minister Adel al-Jubeir said the only solution to the conflict was through a consensus that would achieve the demands of the Syrian people.
"There is no solution to the crisis without a Syrian consensus that would achieve the demands of the Syrian people on the basis of Geneva 1 and (U.N. Security Council) resolution 2254," he said.
The Moscow group of Syrian political activists led by a former deputy prime minister Qadri Jamil said they turned down an invitation to attend the conference, accusing the members of the main HNC opposition of thwarting efforts to set up a single delegation.
"We consider the effort of some of the opposition to exploit the meeting as a platform for their own political ends goes against Saudi Arabia's efforts to form a unified delegation," Jamil said in a statement.
Russia has led an campaign to push for the merger of the Moscow group into a single delegation with the HNC. The main opposition body says it is a stooge of Moscow to sow divisions within their ranks.
The opposition meeting is set to last until Friday, when a joint statement is expected. Several rounds of U.N. talks in Geneva between the Damascus government and the opposition have made little progress since the Syria conflict erupted in 2011.