Riad Hijab, chief coordinator of the Syrian opposition's High Negotiations Committee (HNC), attends a joint statement with French President Francois Hollande (not seen) following their meeting at the Elysee Palace in Paris, France, December 12, 2016. REUTERS/Jacky Naegelen

BEIRUT (Reuters) - The head of the Syrian opposition’s High Negotiations Committee (HNC) resigned on Monday, nearly two years after he was picked to chair the Saudi-backed umbrella group that brings together the armed and political opposition to President Bashar al-Assad.

Riyad Hijab, a former Syrian prime minister under Assad, did not explain his reasons for stepping down in a statement posted on social media.

The HNC has been the main representative of the Syrian opposition since its formation at a meeting in Saudi Arabia in December, 2015, and has taken part in U.N.-led diplomacy aimed at ending the conflict that erupted in 2011.

The war has been going Assad’s way since Russia sent its air force to support him militarily in 2015. The Damascus government has been steadily regaining control of territory, thanks also to the support of Iran-backed forces such as Lebanon’s Hezbollah.

Saudi Arabia is due to host an “expanded” conference for the Syrian opposition this month, aiming to unify its position ahead of more U.N.-backed peace talks, the Saudi state news agency SPA reported last week.

Previous rounds of Geneva peace talks have failed to make headway towards a resolution of the war.