Turkish authorities arrested the suspected killer of two Syrian journalists from the city of Raqa who were opposed to the Islamic State (IS) extremist group and were found beheaded in Turkey in 2015, state media said Monday.

The bodies of Ibrahim Abdul Qader and Fares Hamadi were found beheaded at a friend's house in the southern Turkish city of Sanliurfa in October 2015. The double murder was later claimed by IS.

They were members of the "Raqa Is Being Slaughtered Silently" (RBSS) group which monitors atrocities by IS in its main stronghold in northern Syria.

The killings at the time raised grave concerns over the security of Syrian journalists and activists inside Turkey.

The state-run Anadolu news agency said that investigators had identified the killer as suspected IS member Muaz El Ahsin and that he had moved back into Syria after the killings.

However El Ahsin crossed illegally again into Turkey three days ago and was detained by police, it said. He was then remanded in custody by a court in Sanliurfa on murder charges.

Anadolu said other suspects remain wanted over the killing and police were seeking to apprehend them.