His statement appeared to be the first time Moscow said it was aiding Assad's opposition against ISIS.
More than 100 members of Syria's opposition parties and rebel fighting groups are on board.
Islamic State militants have made more than $500 million from black market oil sales.
It is hoped that the factions can form a united opposition ahead of January peace talks with the government.
Syria said four coalition jets killed three of its soldiers and wounded 13 in the eastern province of Deir al-Zor on Sunday.
For Iran, preventing Assad’s fall is paramount; battling ISIS is of secondary importance.
Kerry added that such a move should not deter opposition groups trying to topple the Syrian government.
We urgently need to eliminate the sectarianism being spread by the Iranian regime.
U.S. Secretary of State John Kerry has said Syria is “weeks” away from a political transition.
France has called for a global alliance against Islamic State.
Kerry also said the war in Syria could be weeks away from a major breakthrough.
Deir Ezzor’s residents remain steadfast in their resistance to ISIS despite the siege and fighting.
Chemical weapons experts say mustard gas was used in a Syrian town where ISIS was battling a rebel group.
“If the international community had instituted the no-fly zone, Aleppo would be free today.”
Cluster bombs are a reliable method for killing noncombatants outside an intended target field.
Putin may have insisted on new Iranian forces before committing his air force.
It will be the first time that Tehran attends an international summit on the four-year-long war.