Sombath Somphone, 62, went missing on December 15, 2012, when he was seen being led away by police in Vientiane after his car was stopped at a checkpoint.
CCTV images later emerged appearing to show him being driven away with two unidentified people.
"We are deeply concerned about his safety and security," the UN working group on enforced or involuntary disappearances said in a statement.
It urged the Laos government to "do its utmost to locate Mr Somphone, to establish his fate and whereabouts, and to hold the perpetrators accountable."
The government has repeatedly denied it had detained or was holding Somphone.
The experts meanwhile highlighted today recent information they said sheds new light on the case.
The missing activist, they said, had been seen a few days after his disappearance inside a police detention centre with his car parked in the police compound.
Two days later, he was reportedly moved to a military camp outside Vientiane, and then transferred again to an unknown location a week later, they said.
"Mr Somphone's disappearance might have a chilling effect on human rights defenders operating in the country, owing to his high profile at the national and international levels," she said in the statement.
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