BEIRUT: A US-led coalition air strike on eastern Syria has killed a founder of ISIS's notorious Amaq propaganda agency, Syrian opposition activists said on Wednesday.

Rayan Mashaal, also known as Baraa Kadek, was reportedly killed in coalition bombardment on the ISIS-held town of Mayadeen, near Syria's border with Iraq, on Monday.

Several Syrian activist Facebook pages, including the prominent media collective Eye on the Homeland, shared the news of his death.

"Mashaal was one of the founders of Daesh's Amaq news agency," Eye on the Homeland said, using the Arabic acronym for ISIS.

"He and his daughter were killed in a coalition air strike on Mayadeen in Deir al-Zor province," the collective wrote.

A Facebook post reportedly written by Mashaal's brother was also being circulated as confirmation of his death.

Neither the US-led coalition nor the Syrian Observatory for Human Rights monitoring group could immediately confirm the news.

Mohammad Khaled, the executive director of opposition news network Aleppo24, told AFP that Mashaal, 31, had been a well-known media activist before joining ISIS.

"I met him in 2012. Baraa (Mashaal) was one of the old revolutionaries of Aleppo," he said.

According to Khaled, Mashaal worked as a media activist in Aleppo until late 2013, when he announced he was moving to "the land of the caliphate" in ISIS-held Al-Bab.

He went on to move to ISIS's Syrian bastion Raqqa, but fled to Mayadeen four months ago as the US-backed Syrian Democratic Forces drew near to the northern city, Khaled said.

"We knew that he was founding this (Amaq) agency in 2014 because he invited all the activists in Aleppo to join him," Khaled said.

Amaq is ISIS's self-styled news outlet.

Its channels on social media application Telegram are often the first place that ISIS publishes its claims of responsibility for attacks around the world.

The US-led coalition has been bombing ISIS targets in Syria since September 2014 and has been heavily bombing Mayadeen since last week.

The Britain-based Observatory said on Friday that at least 80 relatives of ISIS fighters were killed in coalition strikes on the town.

More than 320,000 people have been killed and millions displaced by Syria's conflict, which erupted in March 2011 with protests against Syrian President Bashar Assad.