A UN envoy has accused the army in the Philippines of being "in a state of almost total denial" about a wave of political killings.
Arroyo should do more, says the UN envoy
Many of these killings have been "convincingly attributed to them", said special rapporteur Philip Alston.
One human rights group says since President Gloria Arroyo took power in 2001, 830 activists have been killed.
Mr Alston urged President Arroyo to confront what he called a culture of virtual impunity in the legal system.
The Philippines army, which has been battling communist rebels for almost four decades, has blamed rogue elements for some of the killings.
But Mr Alston said that such explanations were insufficient.
The army "needs to give us precise details and to indicate what investigations and prosecutions have been undertaken in response".
Rights group figures
Although critical of the president, the UN rapporteur stressed that he did not believe that there was any political sanction for the killings.
Alston presented his provisional findings in Manila
"I do not believe that there's a policy at the top designed to direct that these killings to take place," said Mr Alston.
The human rights group Karapatan says that 832 extra-judicial killings since 2001 can be blamed on the security forces. Of these, it says 356 are left-wing activists.
The Philippines armed forces have been fighting a Communist insurgency since 1969, with an estimated 40,000 people killed, and in recent years they have also had to face attacks by Muslim radicals.
But military chief General Hermogenes Esperon denied that extra-judicial killings were used, and said he would prosecute any soldier found doing this, Reuters news agency reports.
Philip Alston has spent 10 days in the Philippines investigating the killings, and is due to present a full report on his findings in three months' time.