World leaders are venting over Syria's civil war but look no closer to agreeing on international military intervention to stop it.

A French official says leaders at a summit of the Group of 20 leading world economies in Russia agreed with President Barack Obama and French President Francois Hollande that chemical weapons had been used in a 21 August attack in Syria, and condemned it.

But many leaders remain in doubt about whether Syrian President Bashar Assad's regime was behind the attack, or Syria's rebels. The US and France are preparing possible military action against Assad, and are trying at the G-20 summit to get backing from other world powers.

The French official, speaking Friday in St. Petersburg, was not authorized to be publicly named according to presidential policy.