Image caption After the murder of Luis Santiago of Ciudad Juarez, Mexico's El Diario, the paper curtailed crime coverage

More than 251 journalists in 13 countries were killed "with impunity" in the past decade, the Committee to Protect Journalists has reported.

Across the world, the unpunished murders lead to self-censorship and press silence, the group reported.

The group singles out Iraq, Somalia, the Philippines and Sri Lanka as the worst environments for journalists

The situation for journalists worsened in Mexico but improved in Russia in 2010, the group wrote in a new report.

"The targeted killing of journalists serves as a silencing message to others, ensuring that sensitive issues are not subjected to public scrutiny," Committee to Protect Journalists Executive Director Joel Simon said in a statement.

"Many journalists who were murdered had been threatened beforehand but were left unprotected. Governments can either address anti-press violence or see murders continue and self-censorship spread."

The New York-based group's report was released the day after the body of Pakistani investigative journalist Saleem Shahzad was found; Mr Shahzad had written about al-Qaeda's infiltration of Pakistan's navy.

In a report released on Wednesday, the Committee to Protect Journalists wrote that murders of local journalists constitute the majority of unsolved cases, and that corruption and dysfunction in law enforcement keep journalists' killers from being brought to justice.

The committee found the killings lead journalists to avoid sensitive topics, quit the profession or flee in order to avoid violence.

In 2010 Russia's standing on the "impunity index" improved, as no journalists were murdered that year and authorities won two murder convictions.

Mexico worsened for the third straight year when news photographer Luis Carlos Santiago, 21, of El Diario of Ciudad Juarez was shot dead in midday in a shopping centre parking lot.

The committee's report covered 13 countries in which the murders of more than five journalists remain unsolved in the past decade. Ranked by unsolved killings proportional to population, those include: