Fewer journalists were imprisoned this year but the number held hostage jumped, media rights group Reporters Without Borders (RSF) said Tuesday in an annual report, with China and Egypt named the worst nations for jailing media workers.

The number of journalists put in prison fell 14 percent in 2015 from last year, to 153, RSF said, describing China as "the world's biggest prison for journalists", followed by Egypt.

Iran and Eritrea were also condemned by the organisation for jailing members of the press.

Fifty-four professional journalists were held hostage in 2015, an increase of 35 percent from last year, the tally said, pointing to Syria as the country with the highest number of reporters in the hands of extremist or criminal groups at 26.

The Islamic State group alone is holding 18 journalists, largely in Syria and Iraq.

"A full-blown hostage industry has developed in certain conflict zones," RSF secretary-general Christophe Deloire said, highlighting Yemen as the newest problem country for reporters, with 33 kidnapped by Houthi militias and Al-Qaeda in 2015, compared with just two the previous year.

"We are very alarmed by the increase in the number of reporters held hostage in 2015. The phenomenon is above all linked to the big surge in abductions of journalists in Yemen," Deloire added.

Lawless Libya had the largest number of journalists reported missing this year, with eight members of the press unaccounted for, and a political climate that "makes it harder to conduct investigations to locate missing journalists".