Sudan must free political detainees and guarantee press freedom to show its commitment to a national dialogue aimed at resolving multiple rebellions, a UN expert said on Tuesday.

Mashood Adebayo Baderin welcomed the June 15 release of Umma Party chief Sadiq al-Mahdi but said another opposition party leader, Ibrahim al-Sheikh of Sudanese Congress, should also be freed.

By releasing Sheikh and all other political detainees, the government would "further demonstrate its good faith and genuine commitment to the national dialogue it has proposed," Baderin, the United Nations independent expert on human rights in the Sudan, told reporters.

Sheikh was arrested on June 8 after a rally at which he -- as Mahdi did before his own arrest -- criticised the actions of the paramilitary Rapid Support Forces (RSF), according to a Sudanese Congress party official.

The RSF have been accused of attacking villages in the Darfur region, which they deny.

Mahdi was held for about one month after his arrest by agents of the National Intelligence and Security Service (NISS).

Mahdi's Umma Party was a key target of the national dialogue which President Omar al-Bashir called in January to resolve multiple crises in the impoverished, war-ravaged nation.

But the arrest of Mahdi, and other actions, raised questions about the regime's commitment to reform.

Baderin said the country's newspapers continue to be subjected to censorship before and after publication, and journalists are forbidden from crossing certain "red lines."

The guarantee of press freedom "is an essential factor for meaningfully facilitating the national dialogue", Baderin said at the close of his latest 10-day mission.

He is to report to the UN Human Rights Council in September.