A deputy of Sudan's Islamist opposition leader Hassan al-Turabi, detained for more than four months, is being investigated by police for "creating war" against the state, a party official said on Saturday.

Ibrahim al-Sonosi and another figure in the opposition Popular Congress Party were arrested at Khartoum airport in December after returning from a trip to Kenya and South Sudan.

"Yesterday they were sent to the police for investigation," after four and a half months of detention by Sudan's intelligence service, said lawyer Hassan Abdullah Al-Hussein, the party's human rights secretary.

He said he was not allowed to meet the suspects and police would not officially talk to him.

Informally, an investigator told him the two party members are accused of "destroying the constitutional system of the state, creating war against the state and creating war against the Sudanese Armed Forces," Hussein said.

He had no other details, including the possible penalty upon conviction.

"They don't have any evidence against them. Our party activities are not hidden," the lawyer said.

In February, Popular Congress leader Turabi accused the intelligence service of bugging the party's offices, and said intelligence agents falsely accused the party of planning a coup in combination with a popular uprising.

Turabi said Popular Congress admits to seeking removal of the 23-year-old Islamist regime of Omar al-Bashir through a popular revolution, but not by force.

Turabi was a key figure in Bashir's regime for a decade.

Many believed Turabi was the real force behind a drive to implement rigorous Islamic practices. Sudan became a notorious refuge for militant Islamists, including Al-Qaeda mastermind Osama bin Laden, and Khartoum was accused of sponsoring terrorism. It was subsequently blacklisted by governments including the United States and Egypt.

Turabi fell out with Bashir after a protracted power struggle, was jailed, and became one of the president's fiercest critics.