The United Nations Special Rapporteur on the situation of human rights in Cambodia on Thursday urged the government to release an opposition leader who has been under de facto house arrest since September last year.

Kem Sokha "remains under detention given the restrictions on his movement, surveillance of his family and friends when they visit and restrictions on who can visit," Rhona Smith said at a press conference in the nation's capital, Phnom Penh, at the end of her official visit that started around the beginning of this month.

The country's authorities refused the UN official access to meet Sokha, who heads the country's main opposition Cambodia National Rescue Party and who spent more than a year in custody after being accused of treason in 2017.

"I reiterate my call for the release of Kem Sokha from detention and the swift conclusion of the investigation or for the charges to be dropped," Smith said in a statement, in which she also urged for progress on the human rights situation in the country in order to "overcome the challenges of the current political situation."

Moreover, Smith urged the government, led by Prime Minister Hun Sen, to fight corruption and ensure the right to peaceful public assembly for the members of the opposition.

Sokha was arrested on Sep. 3, 2017, following the release of a video in 2013 in which he appeared to be boasting to supporters of having the necessary support from international pro-democracy organizations to win the elections.

The arrest came at the request of PM Hun Sen, who has been in power since 1985. He accused Sokha of conspiring with foreigners to overthrow the government, a charge that carries a penalty of between 15 and 30 years in prison.

Sokha's detention was followed by a judicial offensive on his party, which was eventually banned towards the end of 2017, resulting in about a hundred of its top officials fleeing the country.

The repression was extended to the media outlets such as the Cambodia Daily newspaper, which was forced to shut down because of an alleged debt with tax authorities, and the US-financed Voice of America and Radio Free Asia stations, which closed their offices in the country.

"I take this opportunity to call on the government to increase the space for a free press, including space for independent journalists to operate," underlined the special rapporteur.

After winning the elections in July 2018, Hun Sen responded to criticism with several gestures of leniency, including releasing the opposition leader from prison and putting him under house arrest, as well as granting pardons to some 20 imprisoned dissidents.