Hun Sen said though it was ultimately up to the court to decide, Kem Sokha, whose Cambodia National Rescue Party (CNRP) was banned ahead of a 2018 election, should be given special dispensation to leave.

Such overtures to rivals are rare for the self-styled strongman, who has ruled Cambodia for 37 years and has gained notoriety for his harsh treatment of his opponents, most of which are now in exile.

"Going abroad to seek a medical checkup and visit children, there should be no obstacle," Hun Sen told supporters on the sidelines of a summit in the United States.

"The decision whether to allow him to go or not is the complete power of the court. I don't interfere but if I am the court, a judge, I would allow if requested."

Presiding judge Koy Sao could not be reached for comment on Thursday.

The CNRP's dissolution effectively turned Cambodia into a one-party state, allowing Hun Sen's party to win every seat in parliament and influence independent bodies.

Kem Sokha was freed from house arrest in 2019 but his trial is ongoing. He is banned from political activities.

His treason charges stem from accusations he had conspired with the United States to overthrow Hun Sen. Kem Sokha and Washington have refuted that.

Scores of members of the disbanded CNRP are also trial in what the United Nations and human rights groups say is a sham.

Many CNRP members fled overseas, including most of its leadership, leaving behind a country with no functioning opposition.

Pheng Heng, a lawyer for Kem Sokha, said he was not aware of any plan to go abroad, but his client would go if permission was granted. He urged the government to go a step further.

"If they really have a willingness to compromise, they should ask a prosecutor to withdraw the charges," Pheng Heng told Reuters.

Australian Associated Press