Mrs Robinson told the closing session of the UN Human Rights Commission in Geneva that the international war crimes tribunal could investigate both sides in the conflict.

The former president of Ireland also repeated her criticism of the Serbs, saying they had callously mistreated women, children and the elderly.

Mrs Robinson is asking Nato to rethink its campaign

Correspondents say this is the first time she has spoken out against Nato so strongly and is some of the harshest worded criticism to emerge from the UN about the air strike 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," Mrs Robinson said.

"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."

She also called for fresh diplomatic and political efforts to resolve the Kosovo crisis.

"Unless diplomacy succeeds, Kosovo will be thoroughly cleansed of Albanians, while Serbs will, on present performance, be bombed without end," she warned.

"There must be a better way," she said.

"I call for reason to prevail on all sides and for a return to diplomacy and peacemaking."

Trip to Yugoslavia

She told journalists after her speech that she planned to meet Yugoslav President Slobodan Milosevic towards the end of a 12-day trip to the Balkans which begins on Sunday.

Her trip, which starts in Skopje, Macedonia is expected to be followed by stops in Tirana, Sarajevo, Zagreb and Belgrade.

Diplomatic efforts are continuing

Referring to the International Criminal Tribunal for former Yugoslavia, she told the Commission: "Under the tribunal's statute, the prosecutor may investigate war crimes committed by any of the parties to the armed conflict.

"The actions of individuals belonging to Serb forces, the Kosovo Liberation Army (KLA), or Nato may therefore come under scrutiny, if it appears that serious violations of international humanitarian law have occurred," she added.

Mrs Robinson said the passage referring to potential war crime indictments, including Nato officials, was taken from a letter from Louise Arbour, chief prosecutor of the Hague-based tribunal.