The U.N. investigator on torture said on Tuesday the United States had denied his request to visit U.S.-run jails in Iraq and insisted a visit could help clear its legacy of the prison abuse scandal in Abu Ghraib.

Manfred Nowak, United Nations special rapporteur on torture, said he had received credible information the situation had improved at U.S. detention facilities in recent years, but stressed only a visit would allow him to verify them.

An international outcry erupted in 2004 after images of prisoner abuse by U.S. military personnel at Abu Ghraib west of Baghdad, including naked detainees stacked in a pyramid and others cowering before snarling dogs, became public.

"I was a little astonished that the U.S. government is not willing to grant me access because it might perhaps even be in their own interest if I compared different detention facilities," Nowak told a news briefing in Geneva.

"It might also be in their interest in overcoming the legacy of having been criticised so much for torture practices in Abu Ghraib and other detention facilities up to 2004," he added.

Nowak, who has an Iraqi government invitation for his Oct 18-26 planned visit, said he would also expect full access to Iraqi-run detention facilities, although this was still under negotiation. British authorities have agreed to allow him to visit their detainees in Iraq, he added.

At least 30,000 prisoners are held by Iraqi authorities, according to the International Committee of the Red Cross which made its first visit to security detainees held by Iraq's central government last October.

The neutral Red Cross -- whose reports are confidential unlike those of U.N. investigators -- still seeks a wider agreement for access to all prisoners held by Iraq. Sunni Arabs have accused the Shi'ite-led interior ministry of operating torture centres and dungeons holding Sunni detainees.

Nowak also voiced dismay at President George W. Bush's veto last Saturday of legislation passed by Congress that would have banned the Central Intelligence Agency (CIA) from using waterboarding. The U.N. envoy reiterated that the interrogation technique which simulates drowning amounted to torture.

"I think that the (U.S.) government wishes to maintain certain positions of principle which they have taken at the beginning of the so-called war on terror, and if they now would take them back as a government, they would kind of admit that what they had done in the past was wrong," he said.

"I think that the current administration still sticks to its legal position although there is enough evidence that these legal positions are untenable under international law," he said.

Nowak, an Austrian law professor who has served in the independent post since 2004, spoke on the sidelines of the U.N. Human Rights Council. Its 47 member states are holding a four-week session until March 28 to examine abuses worldwide. (Editing by Jonathan Lynn and Matthew Jones)