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.