COLOMBO, Aug 24 (Reuters) - Sri Lanka's northern Jaffna peninsula is one of the most dangerous places in the world for media workers to operate, press freedom groups said on Friday, calling on the government to investigate a spree of murders. Around a dozen mostly minority Tamil journalists and media workers have been killed in Sri Lanka since 2005 as the island slid back towards civil war, rights groups say, seven of those in the Jaffna peninsula, which is cut off from the rest of the island behind Tamil Tiger lines.

Some of the deaths have been blamed on state security forces, some on paramilitaries seen allied to the government and some on the separatist Tamil Tiger rebels. But there have been no convictions.

"Murders, kidnappings, threats and censorship have made Jaffna one of the world's most dangerous places for journalists to work," Reporters Without Borders and International Media Support said in a joint report published on Friday.

"At least seven media workers, including two journalists, have been killed there since May 2006. One journalist is missing and at least three media outlets have been physically attacked. Dozens of journalists have fled the area or abandoned the profession."

Newspapers operating in highly-militarised Jaffna have lost 90 percent of their staff in the past year, while one journalist at Tamil daily Uthayan has not stepped outside his office for 13 months for fear of assassination, the report said.

"Journalists are caught in the crossfire between the security forces, the paramilitaries and the LTTE and live in fear of reprisals for any article, commentary, photo or cartoon they produce," the report added.

Government officials were not immediately available for comment on the report.

Press freedom groups who visited the island in June called on President Mahinda Rajapaksa's government, under pressure over the island's human rights record after a rash of killings and abductions, to invite a United Nations human rights monitoring mission to the island. The government has refused.

"Because of all these problems of violence against the press, there's a real lack of independent news about what's going on because of threats from both sides," said Vincent Brossel, head of Reporters Without Borders' Asia desk.

"It's more than dangerous, it's a place where it's impossible to practice journalism in the normal way," he added. "Of course Iraq is much more dangerous, but if you look at different areas affected by civil war or conflict, it's maybe at the same level as Gaza or Chechnya."

Access to Tamil Tiger-held areas has been restricted since August 2006 for what the government says are security reasons, but which one top official said was to avoid Tiger propaganda being spread.

U.N. Under Secretary General for Humanitarian Affairs John Holmes this month described Sri Lanka as having one of the worst records in the world for humanitarian aid worker safety, citing the killings of around 30 aid staff since January 2006.

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