US Secretary of State John Kerry urged Laotian authorities late Sunday to step up their investigation "without further delay" into the disappearance of a prominent US-educated Laotian community development worker 100 days ago.
Sombath Somphone, the 62-year-old founder of a non-governmental organization campaigning for sustainable development, disappeared in Vientiane while driving home on December 15.
CCTV images showed him being taken away from a police post by two unidentified individuals.
"The United States shares the international community's serious concerns about Mr Sombath's safety and well-being," Kerry said in a statement.
"We call on the Lao government to do everything in its power to account for his disappearance without further delay."
The United States has sought more information from Laos on its investigation into Sombath's disappearance, which has sent jitters through the activist network in the secretive one-party communist state.
Laotian authorities have suggested he might have been abducted over a personal dispute but have denied having any information about his whereabouts.
"We are concerned at the lack of significant information we have received from the Lao government about Mr Sombath's case, despite our offers to assist with the investigation and numerous expressions of concern about Mr Sombath's welfare," said Kerry.
The top US diplomat noted that despite Laos's growing integration into the community of nations, "Mr Sombath's disappearance resurrects memories of an earlier era when unexplained disappearances were common."
"Regrettably, the continuing, unexplained disappearance of Mr Sombath, a widely respected and inspiring Lao citizen who has worked for the greater benefit of all of his countrymen, raises questions about the Lao government's commitment to the rule of law and to engage responsibly with the world," he added.
"We join with countless organizations, governments, journalists and concerned citizens around the world in demanding answers to Mr Sombath's disappearance and urging his immediate return home."
The campaigner, who had earned degrees in education and agriculture from the University of Hawaii, won the 2005 Ramon Magsaysay Award for Community Leadership for his work in poverty reduction and sustainable development in a country that remains one of Southeast Asia's poorest nations.
Daniel Baer, deputy assistant secretary for the State Department's Bureau of Democracy, Human Rights, and Labor, told AFP that Sombath's disappearance has had "a chilling effect" on his activist network.
"For as long as the case remains unresolved and Sombath doesn't come home to his wife, the international community as well as many people here who know and love him will continue to ask questions," Baer noted.
The secretive one-party communist state -- which exerts total control over the media and does not tolerate criticism -- has in recent years gradually given local civil society groups more room to operate.
Sombath's disappearance has sparked an international campaign of solidarity with the activist and his family.
The European Union, the office of the United Nations High Commissioner for Human Rights, members of parliament from Asia and Europe, and numerous international organizations have urged the Laotian government to take all actions necessary to ensure the safe return home of this respected figure.