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The family of US journalist Steven Sotloff, who was beheaded by the Islamic State group, has sued Syria on grounds it provided material support to the terrorist organisation.

In a suit filed Monday in US District Court in Washington, Sotloff's parents and sister seek $90 million in damages from the government of President Bashar al-Assad.

It is not clear how the plaintiffs would collect the money if they win the case.

Sotloff entered Syria in August 2013 from Turkey and was abducted while traveling to the northern city of Aleppo.

About a year later, he was beheaded and a video of the atrocity was released by the Islamic State group.

A few weeks earlier another American journalist, James Foley, was also beheaded by the group.

The Sotloff lawsuit argues that Syria helped create and later assisted the Islamic State group as a "sham opponent" in the Syrian civil war, so as to bolster Assad's negotiating power with Western powers that wanted him removed.

The murder of journalists such as Sotloff helped Assad because it shifted Western attention away from getting rid of Assad to defeating the Islamic State group, with Assad as "a necessary ally," the lawsuit states.

"Syria's material support for Daesh caused the abduction and murder of Steven Sotloff," the lawsuit says, using an acronym for the group that comes from its name in Arabic.

The suit says Syria has provided the jihadist group with financial support, materiel, and military air support, and with direct instructions through embedded intelligence officers.

The suit includes as footnotes quotes from US and British officials asserting that Assad and IS are in fact in collusion financially, even as they ostensibly fight each other.

Assad buys stolen oil from IS militants, and this exchange of cash for crude suits each side, this argument goes.

The lawsuit asserts that Syria can be sued in US federal court. It also notes that Syria is on the US list of countries deemed as state sponsors of terrorism.