At least 500 people were killed in what appeared to be a chemical weapons attack in the outskirts of Damascus. A U.N. inspection team is on the ground in Syria, though it's there to investigate earlier claims of the use of the nerve agent sarin.
Iranian Foreign Minister Mohammad Javad Zarif said in a late Wednesday phone call with Turkish Foreign Minister Ahmet Davutoglu rebel forces carried out the act, because it was unlikely the regime would make such a move with U.N. inspectors in Syria.
"This criminal act had been done by the terrorists, because escalating the crisis in Syria and internationalizing it is in their favor," he was quoted by the semiofficial Fars News Agency as saying.
Russian Foreign Ministry spokesman Alexander Lukshevich struck a similar tone, saying blaming Syrian President Bashar Assad would spoil any chance for peace.
"It all looks like attempts to use any means to persuade the U.N. Security Council to take the side of the opponents of the regime," he was quoted by state news agency RIA Novosti as saying.
Western leaders suggested the Syrian military carried out the attack on the rebel-held area. Russia says Western policies on Syria are biased.
U.N. Secretary-General Ban Ki-moon said he was concerned by the use of chemical weapons. He said Monday the conflict in Syria is a "stark example" of how divisions among members the U.N. Security Council can impede action.