The government of Laos is denying responsibility for the disappearance of a prominent social activist, but human rights groups disagree, saying security camera footage suggests he was detained by Lao authorities.

Sombath Somphone was last seen Saturday in the capital of Vientiane, when his jeep was stopped by authorities outside a police post. Video footage appears to show him being taken away with two unidentified men.

The Lao Foreign Affairs Ministry says a preliminary assessment of the footage suggests the 60-year-old activist may have been “kidnapped because of a personal conflict or conflict in business.”

It said the government has no information on Sombath's whereabouts, but that authorities are “seriously investigating.”

Many rights groups remain unconvinced, saying he is likely in state custody.

Human Rights Watch said Thursday the government in Laos should immediately reveal Sombath's location and release him.

A group of 132 Asian intellectuals wrote a letter Thursday to Lao officials, urging a thorough government investigation into their colleague's disappearance.

The U.S. State Department has also called for the Lao government to make every effort to locate the activist.

Laos, a single-party communist state that does not tolerate dissent, has been criticized in the past for carrying out arbitrary arrests and forced disappearances.

In 2005, Somphone received the Ramon Magsaysay Award for Community Leadership, widely regarded as Asia's Nobel Peace Prize, for his efforts to promote sustainable development.

His friends point out that Sombath's work was not directly political or confrontational and that he was not know to have any enemies.