By Robert Evans

GENEVA (Reuters) - United Nations' torture investigator Juan Mendez said on Tuesday the Obama administration showed no sign of reversing its position and allowing him access to terrorism suspects in long-term detention at the Guantanamo Bay prison camp.

Mendez, whose predecessor was also denied access to Guantanamo prisoners, said the latest Washington response indicated there would be no let up in U.S. insistence he could tour the facility but could not interview detainees.

"We had hoped that there would be a change of position when Barack Obama became president in 2009," Mendez told reporters.

"But to my disappointment, the terms (offered by the United States) were no different."

Under the U.N. rules for such visits, human rights investigators should be granted free access to any prisoners they wish to see without any officials being present, and at a time of their own choosing.

A U.N. treaty defines torture as including not just physical abuse but also inhuman and degrading treatment - like long-term solitary confinement - that can inflict mental anguish on inmates.

His predecessor, Austrian professor Manfred Nowak, long sought permission from the administration of George W. Bush to talk to detainees at the prison. Nowak turned down an invitation in 2004 because he would have had no access to them.

"I am persisting, but I don't expect it to happen any time soon," said Mendez, an Argentine human rights lawyer who now lives in the U.S. capital.

HURDLES

The 67-year-old lawyer, who was imprisoned for his defense of political prisoners during military rule in Argentina in the mid-1970s, has been seeking to arrange a free access visit to Guantanamo since he took up his post in 2010.

Mendez said he was also hoping to visit the U.S. federal maximum security prison ADX at Florence, Colorado, where hundreds of convicts - many jailed on terrorism charges - are held in solitary confinement.

The application was submitted through the U.S. State Department last year but bureaucratic hurdles involving state and federal authorities had not yet been overcome. "It's certainly taking longer than I expected," said Mendez.

He said Russia was also insisting he could only tour prisons there to look into complaints of ill-treatment if he accepted he would have to get clearance from local authorities to visit each prisoner he wanted to see.

"Those are terms we just cannot accept," he told a news briefing on the fringes of a month-long session of the U.N. Human Rights Council.

(Reported by Robert Evans; Additional reporting by Stephanie Nebehay; Editing by Sophie Hares)