Rami Abdel Rahman, the head of the observatory, said most of those killed were from Iraqi nationalities.
Iran is a major ally of Syrian President Bashar al-Assad. Iranian troops and their militia allies from Iraq and Lebanon have been fighting alongside Syrian government troops since the uprising.
This is the second attack on pro-Iranian allies near the Syrian-Iraqi border this month.
On September 9, the watchdog said at least 18 members of pro-Iranian militias were killed when unidentified warplanes targeted military vehicles and arms depots in the al-Bou Kamal area and its outskirts in the eastern countryside of Deir al-Zour.
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