Cambodian police said Tuesday they will investigate the alleged disappearance of a self-exiled Thai activist, denying any involvement in what a rights group claimed was an abduction.

Pro-democracy activist Wanchalearm Satsksit, a sharp critic of the Thai government, was dragged into a car in broad daylight last week in the Cambodian capital Phnom Penh, according to Human Rights Watch, which cited witnesses and security camera footage.

"I would like to confirm that Cambodian authorities and police did not arrest that individual," National Police spokesman Chhay Kim Khoeun told AFP, as fears mounted about the activist's safety.

"We are launching an investigation into it... it's not clear yet at what level this information (about his disappearance) is true."

The announcement came after Cambodia's interior ministry spokesman Khieu Sopheak said last week that the HRW report could be "fake news".

He told AFP on Tuesday that Wanchalearm's two-year visa had expired at the end of 2017, and authorities have no evidence if he left the country.

"If our authorities find him, they would arrest him because he was living here with an expired visa... he could not just simply leave," Sopheak said.

Bangkok has denied any knowledge of Wanchalearm's whereabouts.

Fears are mounting about the safety of self-exiled pro-democracy Thai activist Wanchalearm Satskit, who was allegedly kidnapped in the Cambodian capital Phnom Penh
Fears are mounting about the safety of self-exiled pro-democracy Thai activist Wanchalearm Satskit, who was allegedly kidnapped in the Cambodian capital Phnom Penh AFP / Mladen ANTONOV

Since a May 2014 coup, Thailand has vowed to track down pro-democracy critics, especially those accused of attacking the kingdom's unassailable monarchy.

Junta head Prayut Chan-O-Cha was voted in as civilian premier in 2019 elections, but his administration bears the legacy of the coup, with a cabinet stacked with ex-generals and military allies.

"We will cooperate however we can," Prayut Chan-O-Cha told reporters on Tuesday, referring to the alleged disappearance.

Wanchalearm's family issued a public plea on Sunday begging for his "release", and other pro-democracy activists in Thailand have staged small protests demanding an investigation.

The activist is wanted by Thai authorities for allegedly breaching the Computer Crimes Act and Article 116 in the Thai penal code, which criminalises writing that incites unrest.

He ran an acerbic anti-government Facebook page, where he had cryptically written "Compromise Mode" a few hours before his alleged disappearance.

According to HRW, at least eight prominent Thai activists who fled after the last coup to Laos, Cambodia and Vietnam have subsequently disappeared.

If allowed by Southeast Asian governments, such alleged actions risked turning the region into an "autocrats' heaven", warned Charles Santiago, chairperson of ASEAN Parliamentarians for Human Rights (APHR).

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