The initial assessment of the U.S. government suggests that a nerve agent was used in the weekend chemical attack in Douma, Syria, but further evidence is needed to determine what specific agent was used, U.S. government sources said on Monday.
The sources also said the U.S. government had not yet conclusively determined whether the attack was carried out by President Bashar al-Assad's Syrian government forces.
Syria and its main ally Russia blamed Israel for carrying out an attack on a Syrian air base near Homs on Monday which followed reports of a poison gas attack by President Bashar al-Assad's forces on a rebel-held town.
Israel, which has struck Syrian army locations many times in the course of its neighbour's seven-year-old civil war, has not confirmed nor denied mounting the raid.
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But Israeli officials said the Tiyas, or T-4, air base was being used by troops from Iran and that Israel would not accept such a presence in Syria by its arch foe.
The Syrian Observatory for Human Rights, a British-based monitor, said at least 14 people were killed including some fighters of various nationalities, a reference to Iranian-backed Shi'ite militia members, mostly from Iraq, Lebanon and Iran fighting alongside the Syrian army.
The attack demonstrated the multi-faceted nature of the Syria conflict, which started in March 2011 as an uprising against Assad and now involves several countries and various insurgent groups in a web of alliances.