Marty, who is investigating alleged abuses of detainees on behalf of the Council of Europe, had planned to travel to Guantanamo with Manfred Nowak, the United Nations special rapporteur for torture.
They had wanted to question detainees about reports they were earlier held in secret prisons in Europe.
"The US sent a very short reply saying that they couldn’t accept his request to visit and talk to inmates," a spokesman for the Council of Europe in Strasbourg said.
More than 20 mainly European countries colluded in a web of secret Central Intelligence Agency jails and flight transfers of terrorism suspects stretching from Asia to Guantanamo Bay, Marty said in a report published in June last year.
Pointless
"If I cannot speak freely with detainees — as I understand from the American reply — such a visit would be pointless," Marty said in a statement.
"I am disappointed at this refusal by the US, an observer to the Council of Europe, but my investigation continues."
Marty last year said that European governments had violated human rights treaties by helping Washington carry out "extraordinary rendition" flights, the US practice of transporting detainees to other states for interrogation.
The 46-member Council, which is a human rights watchdog, also charged that Romania and Poland had hosted clandestine CIA camps. Bucharest and Warsaw, however, deny the allegation.
Clandestine detention centres, secret flights via or from Europe to countries where suspects could face torture, or extraordinary renditions would all breach the continent’s human rights conventions.
Criticism in Switzerland
At the end of last month, Swiss Justice Minister Christoph Blocher rejected criticism of contacts between the federal prosecutor’s office and the prison authorities running the Guantanamo camp.
Details had been revealed of moves by the prosecutor’s office to obtain information from Guantanamo detainees about terror suspects held in Switzerland.
However, the government agreed that the proceedings had been above board.
The affair caused an outcry in the Swiss media and from human rights, taking the government to task for what were seen as double standards: criticising the US for violating human rights at the prison, while using the Guantanamo inmates to obtain information for its own ends.
Marty described the Swiss-US contacts as "unacceptable", saying they contravened the European Human Rights Convention.
The all-Swiss International Committee of the Red Cross has said it would welcome any development that leads to a clarification of the future of the detainees at Guantanamo.
It does not believe there is presently a legal regime that appropriately addresses either the detainees’ status or the future of their detention.
The ICRC has been visiting detainees at Guantanamo Bay since January, 2002. There are at present around 380 detainees from about 30 countries.
The Geneva-based organisation has facilitated the exchange of almost 28,000 Red Cross messages between the detainees and their families.