* U.N. talks aimed at pressuring Syrian government, rebels
* EU aid chief says Iran and Russia helping aid effort
* Russia cites "major breakthrough" in Syria's attitude
By Stephanie Nebehay
GENEVA, Dec 19 (Reuters) - Iran and Russia have used their influence on Syrian President Bashar al-Assad's government to get visas for U.N. aid workers and improve access for convoys in Syria, but ...