Philip Alston, an Australian law professor and U.N. special rapporteur on extra-judicial executions, delivered the strong indictment of President Gloria Macapagal Arroyo's government at a news conference after a 10-day investigation.

Local rights group Karapatan has said more than 800 people, most of them left-wing activists, have been murdered or reported missing since Arroyo came to power in 2001. But the military says most of the deaths could be attributed to internal fighting in the communist New People's Army.

Alston said he did not know how many had died, but added: "I am certain the number is high enough to be distressing".

"The impact of even a limited number of killings of the type alleged is corrosive in many ways," he said.

"It intimidates vast numbers of civil society actors, it sends a message of vulnerability to all but the most well-connected, and it severely undermines the political discourse which is central to a resolution of the problems confronting this country."

The government said it was concerned about the killings and the fact it had invited the United Nations to investigate was proof it wanted to act on the matter.

"No right-thinking government or leader will tolerate such things happening and that's the reason why we're looking into it," Executive Secretary Eduardo Ermita told reporters.

Alston laid much of the blame on the military.

"The armed forces remain in a state of almost total denial of its need to respond effectively and authentically to the significant number of killings which have been convincingly attributed to them," he said.

Military chief General Hermogenes Esperon has told Reuters that communist rebels were the top security threat to the nation but said troops did not use extra-judicial killings to counter them.

He said he will prosecute any soldier found doing so.

Alston said he asked Arroyo "to persuade the military that its reputation and effectiveness will be considerably enhanced, rather than undermined, by acknowledging the facts and taking genuine steps to investigate".

He said the military must begin to investigate the killings seriously and "not in a way that simply protects its own officers".

The military's claims of an internal purge within the New People's Army were unconvincing, he said. - Reuters