The number of jailed reporters has hit a 15-year high, with Iran, Eritrea and China holding the most, according to a report released Thursday by the Committee to Protect Journalists.

The New York-based rights group said 179 writers, editors and photojournalists were in jail as of December 1 -- up 34 from 2010.

Nearly half of those jailed were online reporters and 45 per cent of them freelancers.

Iran topped the list of jailers for a second consecutive year, with 42 journalists in prison, followed by Eritrea with 28, China with 27, Myanmar with 12, and Vietnam with nine.

"Independent journalists, who often lack the institutional support necessary to resist legal pressures or defend themselves in court, are bearing the brunt of this unprecedented rise in the incarceration of journalists," CPJ Executive Director Joel Simon said in a statement.

"The media gives voice to the grievances of citizens and helps to hold the powerful to account. Their freedom to report represents our freedom to know."

The group said Iran "maintains a revolving prison door with furloughs and new arrests; subjects prisoners to inhumane treatment; and targets their legal counsel." It said 65 journalists had fled the country since 2009.

Worldwide, most jailed journalists were held for criticizing the state or violating censorship rules, the group said. The vast majority were local journalists held by their own governments.

The latest count does not include journalists who either disappear or are abducted by criminal gangs or militant groups.