Three Spanish journalists have been missing in Syria for more than a week, the Spanish Federation of Journalists announced in a statement issued Tuesday.

It is feared that the journalists, who have been identified as Antonio Pampliega, Jose Manuel Lopez and Angel Sastre, may have been kidnapped by the self-proclaimed Islamic State in Iraq and al-Sham (ISIS) militant group, which remains in control of large swathes of Syrian territory.

ISIS has previously abducted a number of foreign journalists, later posting videos showing their execution, often by beheading.

The group has also occasionally asked for ransoms in exchange for the release of abductees.

Syria has been gripped by violence since the regime of President Bashar al-Assad launched a violent crackdown on anti-government demonstrations that erupted in early 2011.

Tens of thousands have since been killed in more than four years of violence between pro-Assad forces and heavily-armed opposition groups.