Cambodia's Supreme Court has denied bail for an opposition leader charged with treason who is seeking to be released for medical treatment abroad.

The court ruled Friday that Kem Sokha must remain in pretrial detention for his own safety and because the investigation into his case is ongoing. His Cambodia National Rescue Party was dissolved last November by a court ruling on a complaint by Prime Minister Hun Sen's government.

Kem Sokha's case is widely regarded as a political setup by the government to cripple its opponents ahead of a general election this July. The party's dissolution was linked to the treason charge against Kem Sokha, for which he could be sentenced to up to 30 years in prison.

Kem Sokha's lawyers say he suffers from high blood pressure and diabetes, and has fallen sick in prison since being detained last September.

The court said if Kem Sokha is sick, the prison will arrange for a doctor to examine him inside the prison facility.

"If Kem Sokha is not allowed to have medical treatment at a hospital and in case he dies inside the prison, who will take responsibility? Are all of you responsible?" one of Kem Sokha's lawyers, Chea Cheng, asked the court.

The Phnom Penh Municipal Court this month granted a six-month extension for Kem Sokha's pre-trial detention period after the expiration of the initial six months. He has now been denied bail three times.

Kem Sokha was arrested last September on the basis of videos from several years ago showing him at a seminar where he spoke about receiving advice from U.S. pro-democracy groups. The opposition party has denied the treason allegation, saying the charge is politically motivated.

In the past several years the opposition party has faced an onslaught of legal challenges from Hun Sen's government with the support of the courts, which are generally seen as favoring his ruling Cambodian People's Party. Court rulings forced Sam Rainsy, Kem Sokha's predecessor as opposition leader, to remain in exile to avoid prison and pressured him into resigning from his party. Other top opposition party leaders fled Cambodia after Kem Sokha's jailing and the party's dissolution.