Spanish authorities are due to investigate nine Syrian officials over claims of torture and concerning the execution of a Syrian man four years ago, in what is the first criminal case against Syrian security forces accepted by a foreign court.

The case was brought by the deceased man's sister, a Spanish woman of Syrian origin, who says her brother disappeared after being illegally detained in 2013 in Damascus.

She learned of his death after finding a picture of his body in a trove of some 50,000 photographs smuggled out of Damascus by a forensic photographer who fled Syria.

One image of Alhaj Hamdo "shows clear signs of torture," according to the charge sheet against the accused - who include Syria's vice president, intelligence chief and air force intelligence chief.

Investigating magistrate Eloy Valesco ruled Spain did have the jurisdiction to launch the probe. Spanish law allows the prosecution of serious crimes in other countries if there is a Spanish victim - which Velasco said the deceased man's sister, Amal Hag Hamdo Anfalis, could be considered as being.

Velasco has called on Anfalis and the forensic photographer to testify in court in April. He said the alleged crimes could constitute crimes against humanity, war crimes, torture and forced disappearance.

At least 320,000 people have died since the Syrian civil war began in 2011. Activists have tried to use European domestic courts to pursue justice for war crimes, given UN Security Council member Russia blocks the referral of Syria to the International Criminal Court.

Other cases have been filed in Germany and France, but have not yet been accepted by the courts, said lawyer Toby Cadman, who represents a London firm helping Anfalis.

jr/gsw (Reuters, AP, AFP)