US Secretary of State John Kerry urged Lao authorities on Sunday to step up their investigation "without further delay" into the disappearance of a prominent Lao activist 100 days ago.
Sombath Somphone was last seen being taken away from a police post by two unidentified individuals in December.
"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.
Lao 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 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.
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.