Responding to reports that Marines allegedly killed several Iraqi prisoners of war, Naval Criminal Investigative Service spokesman Ed Buice said in a statement a probe was now under way.

"NCIS can confirm that it is conducting an investigation into credible allegations of wrongdoing made against US Marines concerning actions said to have taken place in Fallujah Iraq in the fall of 2004," the statement said.

"The Naval Criminal Investigative Service is a fact-finding organization and information gathered will be presented to the appropriate authority who will decide whether further action is warranted."

No further information about the nature of the allegations was revealed. Reports in the North County Times, a local daily newspaper published close to the Marines' massive southern California base at Camp Pendleton, near San Diego, said the probe related to deaths of up to eight Iraqi insurgents.

Multiple sources cited by the newspaper said the inquiry was studying the conduct of between five to 10 Marines involved in fierce fighting for control of Fallujah three years ago.

The newspaper cited a source with knowledge of the investigation as saying that around 20 current and former soldiers had been interviewed and that several had been read their rights, an indication that criminal charges may eventually be laid.

The report said the eight insurgents were shot dead after being captured on November 10 2004. Under military law, the killing of a captured enemy combatant who does not pose a threat is treated as murder.

The probe is the third high-profile war crimes case involving Marines from Camp Pendleton.

Seven Marines face charges ranging from murder to dereliction of duty in connection with the killing of 24 Iraqi civilians in the town of Haditha in November 2005.

In a separate incident, seven Marines and a Navy medic have faced charges over the murder of an Iraqi civilian in the town of Hamdania in April last year. Five have already been sentenced in connection with the case.