Mrs Mary Robinson has denounced both "the pattern of ethnic cleansing" by Serb forces in Kosovo, and NATO's bombing for killing "large numbers of civilians".
In a hard-hitting final speech at the end of the annual United Nations Commission on Human Rights, Mrs Robinson urged the NATO alliance to adhere to "the principle of proportionality" and "weigh the consequences" of its bombing campaign on Yugoslav civilians.
"Unless diplomacy succeeds, Kosovo will be thoroughly cleansed of Albanians while Serbs will, on present performance, be bombed without end. There must be a better way. I call for reason to prevail on all sides and for a return to diplomacy and peace-making.
"What we are in effect seeing is that war-making has become the tool of peace-making," she added.
Mrs Robinson, citing crimes by Serb forces including killings, rape and forced expulsions of ethnic Albanians from Kosovo, said it was "hard to avoid the conclusion of a pattern of ethnic cleansing carried out with cold-blooded determination".
She also criticised the NATO bombing campaign.
"In the NATO bombing of the Federal Republic of Yugoslavia, large numbers of civilians have incontestably been killed, civilian installations targeted on the grounds that they are or could be of military application and NATO remains sole judge of what is or is not acceptable to bomb," she declared.
"In this situation, the principle of proportionality must be adhered to by those carrying out the bombing campaign.
"It surely must be right to ask those carrying out the bombing campaign to weigh the consequences of their campaign for civilians in the Federal Republic of Yugoslavia.
"It surely must be right for the Security Council of the United Nations to have a say in whether a prolonged bombing campaign in which the bombers choose their targets at will is consistent with the principle of legality under the charter of the United Nations.
"Regrettably, conflict victimises innocent people on all sides, Serbs as well as Kosovars, and other nationalities, too. It is therefore all the more crucial and pressing that diplomacy and peace-making be stepped up to bring about peaceful resolution of the situation respectful of human rights precepts."
Tomorrow Mrs Robinson begins an 11-day visit to the Balkans, starting in Skopje, the capital of Macedonia.
Her report to the Commission was based on testimony from refugees fleeing the conflict and gathered by her 10 human rights monitors deployed in Albania, Macedonia and Montenegro.