BANGKOK/PHNOM PENH (Reuters) - Thai immigration police have detained the leader of a Cambodian opposition group based in Denmark, Human Rights Watch (HRW) said on Thursday, while a Cambodian official said the government was in discussion with Thailand on the man’s extradition.

The detention comes ahead of a July general election that Prime Minister Hun Sen’s Cambodian People’s Party (CPP) looks set to win after the main opposition Cambodia National Rescue Party (CNRP) was dissolved by the Supreme Court last year.

Sam Serey, head of the opposition Khmer National Liberation Front (KNLF), was arrested at an immigration center north of the Thai capital, Bangkok, while he was trying to get his Thai visa extended, the New York-based rights group said.

“Our main concern is the safety of Sam Serey if he is deported to Cambodia,” Sunai Phasuk, a senior Thailand researcher at Human Rights Watch, told Reuters in Bangkok.

“Prior to this we have seen members of the KNLF being mistreated by the Cambodian authorities when detained. The Thai government must consider international law as it proceeds.”

In 2016, a Cambodian court sentenced Sam Serey in absentia to nine years in prison for plotting an attack. This month, Hun Sen accused Sam Serey and his group of plotting attacks in Cambodia, calling him a “traitor”.

Sunai said Sam Serey was arrested by Thai police because he had been “blacklisted” by Cambodia over the bomb plots.

A spokesman for Thailand’s national police did not reply to Reuters’ request for comment.

A spokesman for Cambodia’s Interior Ministry, Khieu Sopheak, said the government had been in touch with Thailand to discuss Sam Serey’s extradition.

“We asked that he be deported to Cambodia,” he told Reuters.

Thailand has frequently acquiesced when Cambodia has asked it to repatriate citizens with criminal convictions or those it considers a threat to national security.

In February, a woman who threw a shoe at a billboard depicting Cambodian Prime Minister Hun Sen was forcibly sent from Thailand to Cambodia, where she is serving a two-year prison sentence.

The opposition CNRP was dissolved after it was found guilty of plotting to overthrow the government with the help of the United States, an accusation denied by both.

Ahead of the July vote, the CPP has stepped up prosecutions of critics and political opponents. Media outlets seen as critical of the government have also been forced to shut.