U.S. Secretary of State Hillary Clinton is calling on Laos to investigate the disappearance of a prominent social activist and reunite him with his family.

Human rights groups say Sombath Somphone, a campaigner for a sustainable development in Laos, was detained in the capital Vientiane a month ago and that he is in government custody.

In a statement Wednesday, Clinton urged the Lao government “to pursue a transparent investigation of this incident and to do everything in its power to bring about an immediate and safe return home to his family.”

Lao officials say they have been investigating Sombath's disappearance, but could not find significant leads. The government has suggested that Sombath's disappearance is a result of a personal dispute.

The 60-year-old received the prestigious Ramon Magsaysay Award for community leadership in 2005 and worked to promote education and development in poverty-stricken Laos.

The landlocked communist country has little tolerance for dissent.