GENEVA: The United Nations' torture investigator blasted the White House for defending the use of waterboarding on Wednesday, and urged the U.S. government to give up its defense of "unjustifiable" interrogation methods.

"This is absolutely unacceptable under international human rights law," said Manfred Nowak, the U.N. special rapporteur on torture. "Time has come that the government will actually acknowledge that they did something wrong and not continue trying to justify what is unjustifiable."

On Tuesday, the Bush administration acknowledged publicly for the first time that waterboarding was used by U.S. government questioners on three terror suspects. Testifying before Congress, CIA Director Michael Hayden said the suspects were waterboarded in 2002 and 2003.

Nowak, who has clashed with the U.S. over his failed efforts to investigate at Guantanamo Bay, said he has received more allegations of waterboarding. But he said he did not have proof to back up those allegations, partly because the U.S. will not allow him to speak with high-level terror detainees who were previously held in CIA-run secret prisons.

"If it concerns secret places of detention, it is very difficult to prove," Nowak told The Associated Press by telephone from Vienna, Austria. He added that all allegations of waterboarding were from the "early years" of the war on terror, consistent with Hayden's testimony.