Buenos Aires - A survivor of the Argentine military dictatorship who is serving as a witness in trials against perpetrators of human rights abuses during that era has been released by kidnappers nearly 30 hours after being taken captive in an apparent attempt to silence him.

Argentine human rights activist Juan Evaristo Puthod, a survivor of the last dictatorship in the South American country from 1976-83, went missing on Tuesday prompting an intense search effort.

Puthod appeared bruised and exhausted at a hospital after unknown kidnappers took him hostage in the city of Zarate, 80 kilometres north of Buenos Aires.

There were no official reports on the crime on Thursday.

Argentine President Cristina Fernandez de Kirchner had earlier expressed her "worry over the disappearance."

Puthod, 49, had been held and tortured for years under the dictatorship, severely damaging his health. He lost an eye and had suffered a heart attack just 20 days before the kidnapping.

He is not the first such witness to be kidnapped. In September 2006, Jorge Julio Lopez - a key witness in the trial of Miguel Etchecolatz, a former police high official during the dictatorship who was condemned to life in prison for crimes against humanity - went missing and was never found.

Human rights groups have complained that police and justice officials have not done enough to find out what happened to Lopez.

In late 2006, another human rights witness, Luis Gerez, was kidnapped and mistreated. After 48 hours, he was found by police. The case has not been closed.

Under former Argentine president Nestor Kirchner (2003-2007) legislation that granted an amnesty to police and military officers was revoked. Since then, scores of alleged human rights violators are again coming before the courts.

Some 30,000 people are believed to have been killed by the 1976-83 dictatorship in Argentina. Most of the bodies have not been found. (dpa)