The latest strikes came on the third day of a US-led air campaign aimed at rolling back the Islamic State group in Syria, and appeared to be aimed at one of the militants' main revenue streams.
The US has been conducting air raids against the group in neighbouring Iraq for more than a month.
The Islamic State group captured most of Syria's largest oil fields earlier this year and is believed to be partly funding its operations by smuggling oil out and selling it on the black market.
At least 14 militants were killed, said the Observatory, which gathers information from a network of activists on the ground. Another five people who lived near one of the refineries in the northeastern Hassakeh province were also killed, the Observatory said, adding that they were likely the women and children of the militants.
The planes came "with a terrifying sound and red lights before the explosions," one activist wrote, documenting each explosion.
The militants meanwhile freed at least 150 people from a prison in their de facto capital of Raqqa in northeastern Syria, fearing more strikes, according to activists there.
Other strikes hit checkpoints, compounds, training grounds and vehicles of the Islamic State throughout the territory they hold along the Euphrates River in northern and eastern Syria, with strikes hitting near the Turkish and Iraqi borders.
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