From left, Russian Foreign Minister Sergei Lavrov, Secretary of State John Kerry, Saudi Foreign Minister Adel al-Jubeir and Turkish Foreign Minister Feridun Sinirlioglu stand together before a meeting in Vienna, Austria, Oct. 29, 2015.

The foreign ministers of the United States, Russia, Turkey and Saudi Arabia met Thursday in Vienna to work through their differences on the eve of talks on the war in Syria.

U.S. Secretary of State John Kerry and ministers Sergei Lavrov, Feridun Sinirlioglu and Adel al-Jubeir form a core group spearheading a dialogue between around 20 countries.

Neither Syria’s main political opposition body nor representatives of the armed opposition have been invited to international talks on the country’s war, an opposition politician and a rebel leader said.

The Syrian government in Damascus has meanwhile yet to issue any official comment on the meeting in Vienna on Friday that will bring together about a dozen countries including Saudi Arabia and Iran, which back opposing sides in the conflict.

The Syrian opposition has objected to Iran’s participation in the talks – the first time it has attended such a meeting on Syria – because of its military support for President Bashar al-Assad.

George Sabra, a member of the Syrian National Coalition, told Reuters the failure to invite Syrians showed a “lack of seriousness”. Asked whether the coalition had been invited to the talks, he said: “That didn’t happen.”

“It is a big weak point in the meeting, because it will discuss the issues of the Syrians in their absence.”

Tehran and Moscow have provided crucial support to Assad’s government, while his regional opponents including Saudi Arabia, Qatar and Turkey demand his departure.

Assad has stuck by his long-held position that “eliminating terrorism” must come first, after Moscow called on Syria to prepare for parliamentary and presidential elections. Assad says all the groups fighting him are “terrorists.”

Bashar al-Zoubi of the Yarmouk Army, a group affiliated to the Free Syrian Army, told Reuters that representatives of the armed opposition had not been invited to the Vienna meeting.

“Iran is part of the problem and not the solution, and its participation in the meeting will prove that to the world,” Zoubi said. “This meeting was accepted by Saudi Arabia and Turkey to expose Iran.”

The coalition’s Sabra reiterated his objection to Iran’s participation, saying it could not play a mediation role. “It’s officers are fighting every day on Syrian fronts,” he said.

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