The Britain-based monitoring group said the strike was carried out by the US-led coalition in an area south of Albu Kamal which has repeatedly been attacked by the Islamic State jihadist group.
“At least three vehicles were destroyed by the strike,” said Observatory chief Rami Abdel Rahman.
He said that the 12 pro-regime fighters killed in the incident were not Syrians but he could not provide more details on their identity.
Syrian state media reported the strike earlier but said it only caused material damage.
The US-led coalition which is also battling IS in the remote area in eastern Syria has not yet commented on the reports.