MEXICO CITY/VERACRUZ (Reuters) - Three successive Mexican presidents have failed to halt a cycle of violence against journalists and impunity for their killers that is stifling freedom of expression and threatening democratic stability, a watchdog group said on Wednesday.

The Committee to Protect Journalists (CPJ) urged President Enrique Pena Nieto to prioritize protection for the media in the last year of his government, after at least 21 journalists were slain in the past decade with “complete impunity.”

The violence has created an environment of fear that goes beyond journalism, CPJ’s Americas program director, Carlos Lauria, said, speaking on the same day that the United Nations designated as World Press Freedom Day.

“It’s inhibiting Mexicans from openly debating the problems that afflict society and indirectly affecting the stability of the country’s democracy,” Lauria said during a presentation of a CPJ report on Mexico in the drug-ravaged state of Veracruz.

Mexico, where battles among drug cartels have left tens of thousands dead, has the sixth worst record in the world for resolving the murders of journalists, according to the CPJ.

Despite promises of action by Pena Nieto and his predecessors Felipe Calderon and Vicente Fox, Mexico’s impunity rating has more than doubled since 2008, it added. The CPJ’s impunity index is based on unsolved journalist murders as a percentage of a country’s population.

Four journalists have been killed in Mexico in recent weeks, prompting one newspaper in the northern city of Ciudad Juarez to close down citing a lack of protection from violence.

A special prosecutor’s office tasked with crimes against freedom of expression has failed to produce convictions needed to deter attacks, the CPJ said.

“The pursuit of justice has failed categorically,” the CPJ said in its report. “The system seems to be corrupt down to its very foundation; either that or it’s simply incapable of achieving justice.”

Veracruz, where more journalists have been murdered than the rest of Mexico, is now the deadliest region for the media in the Western hemisphere, the CPJ said.

At least six journalists were murdered in retaliation for their work in Veracruz and three more went missing during the 2010 to 2016 administration of former Governor Javier Duarte, recently detained in Guatemala, the CPJ said.

The group said it was investigating at least 11 more cases in the state to determine if journalists were murdered over their work.

Veracruz is now home to “silence zones, where journalists no longer investigate, where information of public interest is no longer published,” said CPJ’s Mexico correspondent, Jan-Albert Hootsen. “The situation of impunity needs to be solved.”