Sudan's state security agency on Sunday ordered a counter-insurgency unit accused of abusing civilians in the western region of Darfur to deploy around the capital Khartoum.

The move comes as elements of the National Intelligence and Security Service (NISS) engage in what an analyst said is a power play, reflected in their arrest on Saturday of former prime minister and leading opposition figure Sadiq al-Mahdi.

Sudanese authorities have refused to register a political party based on the ideas of a peaceful Muslim activist who was hanged for apostasy, his daughter said on Sunday.

Asma Mahmoud Muhammad Taha had sought official permission to revive the Republican Party of Mahmoud Muhammad Taha, who was hanged on January 18, 1985 under the regime of former president Jaafar Nimeiri.

The arrest of Sudan's former prime minister and leading opposition figure Sadiq al-Mahdi undermines talks aimed at finding a way out of the country's multiple crises, observers said on Sunday.

Mahdi was arrested for alleged treason by agents of the National Intelligence and Security Service (NISS) from his home on Saturday night, officials of his Umma Party said.

Sudan's state security arrested opposition leader and ex-premier Sadiq al-Mahdi for treason on Saturday, party officials said, placing in doubt efforts at governmental reform in the impoverished, war-torn and politically divided country.

By arresting Mahdi, a veteran of five decades on Sudan's political scene, the regime of President Omar al-Bashir "retreats from all promises about dialogue and goes back to square one", Sara Najdallah, secretary general of Mahdi's Umma Party, told reporters at a press conference late Saturday.

Sudan's state security arrested on Saturday opposition leader and ex-premier Sadiq al-Mahdi, his secretary said, after he reportedly accused a counter-insurgency unit of rape and other abuses of civilians in Darfur.

"At 8:45 pm (1745 GMT) a number of state security officers came to the home of imam Sadiq Al-Mahdi with a warrant, and they arrested him," his secretary, Mohammed Zaki, told Agence France Presse.

A Sudanese former prime minister appeared before prosecutors for questioning Thursday after he reportedly accused a counter-insurgency unit of rape and other abuses of civilians in Darfur.

The National Intelligence and Security Service (NISS) filed a criminal complaint against Umma Party leader Sadiq al-Mahdi after he made the comments about the Rapid Support Forces (RSF).

Fighting has intensified between troops and rebels in Sudan's South Kordofan region, adding to humanitarian needs in an area where more than one million people were already affected, the U.N. said on Thursday.

The fighting that began three years ago "has intensified in April-May," the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) said in its weekly bulletin.

A suspect wanted over the deadliest-ever attack in Nigeria's capital -- a blast blamed on Boko Haram Islamists -- has been arrested in Sudan, a source close to the case said Thursday.

Aminu Sadiq Ogwuche was detained on Tuesday as he tried to get a visa from the Turkish embassy in central Khartoum, the source told Agence France Presse, requesting anonymity.

A Sudanese judge on Thursday sentenced a Christian woman to hang for apostasy, in a ruling which Britain denounced as "barbaric."

Born to a Muslim father, the woman was convicted under the Islamic sharia law that has been in force in Sudan since 1983 and outlaws conversions on pain of death.

Leaders of a Sudanese counter-insurgency unit on Wednesday dismissed accusations that their men committed abuses in Darfur.

"We didn’t loot. We didn’t burn any villages. We didn’t rape," General Abbas Abdelaziz, who heads the Rapid Support Forces (RSF), told reporters.