KHARTOUM (AFP) - Sudan on Sunday severed diplomatic ties with Chad, accusing Ndjamena of backing a first Darfur rebel assault on Khartoum, and slapped a multi-million dollar price on the head of the alleged mastermind. The government said it had repulsed the assault by the Justice and Equality Movement (JEM), allegedly backed by Ndjamena, which saw the insurgents reach Khartoum's outskirts with the declared intent of toppling the regime. "We are forced to sever diplomatic relations with this regime" in Chad, President Omar al-Beshir said on state television following the attack on the capital's twin city of Omdurman just across the river Nile. "We place the entire responsibility for this attack on Chad," he said, dressed in his field marshal's fatigues. Chad said it regretted Khartoum's decision, denied any involvement in the attack and condemned a raid on the Chadian embassy. "Chad can only take note of this hasty decision with regret," said the government in a statement. Uniformed Sudanese men ransacked the Chadian mission in Khartoum, taking away documents and computer equipment, it added. A senior official in the military command told the state SUNA news agency that 123 million dollars would be paid to anyone who arrests JEM leader Khalil Ibrahim or provides information on his whereabouts. Beshir convened an emergency session of the National Security Council, which included Salva Kiir, the leader of south Sudan that fought its own two-decade civil war until reaching a power-sharing agreement with Khartoum. Government forces were on Sunday hunting down remnant rebel forces all over the capital and in neighbouring states, rounding up arms and explosives. The official SUNA news agency said the Sudanese military had killed a leading JEM commander, had chased down, fought and wiped out a 45-man rebel force 50km from western Omdurman and arrested 300 rebels. Omdurman remained under curfew but restrictions were lifted elsewhere. The Egyptian news agency MENA said Khartoum international airport was closed and commercial airlines told passengers that flights to Sudan were cancelled. There were no clear indication from either side on casualty numbers. JEM's deputy chief of staff Suleiman Sandal said that his forces had taken Omdurman but were having trouble with the urban fighting environment having come from the desert of Darfur, and had suffered deaths and injuries. Foreign ministry official Ali Yousif said that Sudan had evidence of communication between the rebels, the Chad government and the Chadian embassy in Khartoum.