State media said Abdel Aziz el-Nur Ashr was the half brother of Khalil Ibrahim, the leader of the rebel Justice and Equality Movement (JEM). A security source said Ashr had been acting as the insurgents' field commander.

If confirmed, Ashr would be the most senior rebel arrested since the surprise attack on May 10. State media said he was arrested late on Wednesday in the town of New Halfa, east of Khartoum and near the border with Eritrea.

A rebel commander said that he was still trying to confirm the report, but that it could be true because Ashr was among a number of officers who had stayed behind close to the capital after the attack.

"When we fight we know we can be captured or killed. We expect this," said Suleiman Sandal. "If he has been captured he must be treated as a prisoner of war, under the Geneva convention."

Sandal said Ashr was the commander of JEM forces in Eastern Sudan. JEM has the strongest military force of all the rebel groups in the western Darfur region. It has a national agenda, saying it wants to end government neglect across Africa's largest country.

JEM forces crossed hundreds of miles (km) of desert and scrubland to strike the suburb of Omdurman on May 11, the first time that rebels from Sudan's far flung districts had managed to reach the capital in decades of conflict.

Government forces stopped the rebels at a bridge into central Khartoum. Army officers later said more than 200 rebels, soldiers and civilians had died in the attack

Human rights groups have accused Khartoum of carrying out mass arrests and torturing suspects. The government denies the accusations.

U.N. children's agency UNICEF said the Sudanese government had promised it access to more than 80 children, some as young as 10, detained after the Khartoum attack. The government said JEM had used the children as soldiers. JEM denied that.

International experts estimate that some 200,000 people have died and 2.5 million been driven from their homes since mostly non-Arab rebels took up arms in Darfur in 2003. Khartoum says only 10,000 people have been killed.