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Air strikes launched by the US-led coalition on oil refineries operated by the Islamic State group (IS, formerly ISIS) in northern Syria have killed 30 people, the Syrian Observatory for Human Rights (SOHR) reported on Monday.

Two strikes struck oil facilities in the IS stronghold of Raqqa on Sunday, and most of the dead are believed to be IS militants.

Many of the coalition raids have targeted oil facilities, a major source of income for IS, with some estimates saying that the group earns more than $1 million per day from oil sales on the black market.

The strikes came a day after it was announced that the European Union was adding Syrian businessmen believed to be facilitate oil deals between IS and the regime of Syrian president Bashar al-Assad on to its sanctions list.

Facilities such as a gas plant in Tabqa, northern Syria, are believed to be among those located in IS-held territories that continue to be run by Assad officials, under secret deals between the two sides.

Meanwhile, also on Sunday, coalition air strikes targeted al-Qaeda's Syrian affiliate al-Nusra Front in Idlib, northwest Syria.

An al-Nusra Front base in Atmeh, near the Turkish border, was struck, leaving nine dead, according to the SOHR.

Al-Nusra have been moving in recent months to attack rival opposition groups and establish their supremacy in Idlib, with Zahran Alloush's Jaysh al-Islam and the US-backed Hazzm Movement pushed out of the province.

Jaysh al-Islam have moved their operations to the Damascus suburbs and Ghouta in particular, where regime warplanes struck on Sunday, killing 13 people, including two children, and wounding more than 50.

The SOHR, a monitoring group based in the UK, had previously reported 11 dead.