Major international news organisations have sent a letter to the leadership of the armed opposition in Syria, calling for urgent action against rebel groups increasingly targeting journalists for kidnappings, Belfast Telegraph reported.

The letter, signed by 13 news organisations including the BBC, is in response to a sharp rise in the kidnapping of journalists while on assignment in opposition-held areas in northern Syria. The widespread seizure of journalists is unprecedented and has so far been largely under-reported by news organisations in the hope that keeping the kidnappings out of public view could help with negotiating the captives' release.

The scale of the abductions - more than 30 are believed to be currently held - and the lack of response to individual mediation efforts have encouraged some families and employers to speak out.

Most kidnappings since the summer have taken place in rebel-held territories, particularly in chaotic northern and eastern Syria, where militant al Qaida-linked groups hold influence. Among the most dangerous places are the north-eastern city of Raqqa, which was taken over by al Qaida militants shortly after it became the first city to fall entirely into rebel hands; the eastern Deir el-Zour province; the border town of Azaz; and the corridor leading to Aleppo, once a main route for journalists going into Syria.

"As long as kidnappings are permitted to continue unabated, journalists will not be willing to undertake assignments inside Syria, and they will no longer be able to serve as witnesses to the events taking place within Syria's borders," the letter said.

Signatories to the letter are the BBC, Associated Press, Agence France Presse, Reuters, the New York Times, the Washington Post, the Wall Street Journal, Atlantic Media, the Economist, Getty Images, the Guardian, the Los Angeles Times, the Telegraph and El Mundo.

The open letter is being sent to the leadership of the Western-backed mainstream Free Syrian Army and to individual armed groups including the Islamic Front, an umbrella organisation of six of the most powerful brigades in Syria.

Syria's rebels are a disparate group of brigades and battalions, increasingly dominated by Islamic extremists, including al Qaida-linked groups such as the Islamic State of Iraq and the Levant and Jabhat al-Nusra.