LONDON, Sept 28 (Reuters) - The conduct of U.S. troops in Iraq, including increasing detention and accidental shootings of journalists, is preventing full coverage of the war reaching the American public, Reuters said on Wednesday.

In a letter to Virginia Republican Sen. John Warner, head of the Senate Armed Services Committee, Reuters said U.S. forces were limiting the ability of independent journalists to operate. The letter from Reuters Global Managing Editor David Schlesinger called on Warner to raise widespread media concerns about the conduct of U.S. troops with Defense Secretary Donald Rumsfeld, who is due to testify to the committee on Thursday.

Schlesinger referred to "a long parade of disturbing incidents whereby professional journalists have been killed, wrongfully detained, and/or illegally abused by U.S. forces in Iraq."

He urged Warner to demand that Rumsfeld resolve these issues "in a way that best balances the legitimate security interests of the U.S. forces in Iraq and the equally legitimate rights of journalists in conflict zones under international law".

At least 66 journalists and media workers, most of them Iraqis, have been killed in the Iraq conflict since March 2003.

U.S. forces acknowledge killing three Reuters journalists, most recently soundman Waleed Khaled who was shot by American soldiers on Aug. 28 while on assignment in Baghdad. But the military say the soldiers were justified in opening fire.

Reuters believes a fourth journalist working for the agency, who died in Ramadi last year, was killed by a U.S. sniper. "The worsening situation for professional journalists in Iraq directly limits journalists' abilities to do their jobs and, more importantly, creates a serious chilling effect on the media overall," Schlesinger wrote.

"By limiting the ability of the media to fully and independently cover the events in Iraq, the U.S. forces are unduly preventing U.S. citizens from receiving information...and undermining the very freedoms the U.S. says it is seeking to foster every day that it commits U.S. lives and U.S. dollars," the letter said.

"SPIRALING OUT OF CONTROL"

Schlesinger said the U.S. military had refused to conduct independent and transparent investigations into the deaths of the Reuters journalists, relying instead on inquiries by officers from the units responsible, who had exonerated their soldiers.

The U.S. military had failed even to implement recommendations by its own inquiry into one of the deaths, that of award-winning Palestinian cameraman Mazen Dana who was shot dead while filming outside Abu Ghraib prison in August 2003. Schlesinger said Reuters and other reputable international news organisations were concerned by the "sizeable and rapidly increasing number of journalists detained by U.S. forces".

He said most of these detentions had been prompted by legitimate journalistic activity such as possessing photographs and video of insurgents, whichU.S. soldiers assumed showed sympathy with the insurgency.

In most cases the journalists were held for long periods at Abu Ghraib or Camp Bucca prisons before being released without charge.

At least four journalists working for international media are currently being held without charge or legal representation in Iraq. They include two cameramen working for Reuters and a freelance reporter who sometimes works for the agency.

A cameraman working for the U.S. network CBS has been detained since April despite an Iraqi court saying his case does not justify prosecution. Iraq's justice minister has criticised the system of military detentions without charge.

Schlesinger's letter said: "It appears as though the U.S. forces in Iraq either completely misunderstand the role of professional journalists or do not know how to deal with journalists in a conflict zone, or both."

Reuters and other media organisations in Iraq had repeatedly tried to hold a dialogue with the Pentagon to establish appropriate guidelines on how to safeguard journalists. These efforts had failed "and the situation is now spiraling out of control", Schlesinger said.

He asked Warner to question Rumsfeld specifically about the rules of engagement towards professional journalists, the failure to hold independent investigations into shooting incidents and to ask what was the guidance to U.S. forces on how to distinguish legitimate journalists from insurgents.