The report by the New York-based Committee to Protect Journalists, or CPJ, said US troops had killed 13 journalists since the US-led war started in March 2003, and at least 40 other reporters had been killed in the conflict.
"Several of the 13 deaths suggest indifference by US soldiers to the presence of civilians, including members of the press," the CPJ said in a statement.
The report also examined the deaths of 21 media support staff who were killed in Iraq, two by US forces.
It said that in most cases, the US military had either failed to investigate journalists' deaths or it had not made its inquiries public.
Pentagon spokesman Eric Ruff said he had not seen the report, but rejected the CPJ charges.
"The US military investigates every credible report of a journalist's death in which US forces may have been involved. To suggest otherwise simply does not reflect the facts," Ruff said.
The US military has been reluctant
to probe journalists' killings
"The findings from the few investigations that have been released have not credibly addressed questions of accountability for shooting deaths, and whether US forces are taking necessary measures to differentiate between combatants and civilians in conflict areas," the report said.
In the most recent case, Reuters soundman Waleed Khaled was shot by US forces as he was driving with a colleague on an assignment. The US military has said the action of its troops was "appropriate" in the August incident.
Reuters cameraman Haider Kadhem, 24, like Waleed an Iraqi, was slightly wounded but survived in the passenger seat of the car, only to be detained for three days by US troops.
The CPJ report said the US military had not yet published its report on that incident.
"By failing to account for all shooting deaths and demonstrate that it is taking steps to avoid future tragedies, the US military shows an alarming disregard for the safety of civilians, including journalists covering this conflict," CPJ Executive Director Ann Cooper said.