Sudan Sunday announced it had broken off diplomatic relations with Chad over a row about Chad's alleged support for rebels from Darfur launching an attack on Sudan's capital, Khartoum.

Sudan's President Omar al-Bashir accused Chad of being behind the attack on the Khartuom suburb of Omdurman.

'These forces are all basically Chadian forces supported and prepared by Chad and they moved from Chad under the leadership of (rebel leader) Khalil Ibrahim,' Bashir said.

An overnight curfew imposed on Khartoum was extended until further notice.

The African Union (AU) said in a statement released in the Ethiopian capital Addis Ababa that it was 'deeply concerned' about the spread of the Darfur conflict to Khartuom.

Chad and the Justice and Equality Movement (JEM) rebels have both denied any cooperation in launching attacks on the Omdurman suburb, where the rebels say they are in control - a claim denied by the Sudanese government.

JEM Legislative Council Chairman Eltahir Adam Elfaki, told the BBC from Libya that the group was 'self-dependent - it is a force that built itself from the equipment of the Sudan government following our activities against the Sudan government.'

The African Union warned Sunday that the rebel attacks would only further complicate the efforts to find a political solution to the Darfur solution.

It called on all parties in the conflict to avoid further escalation of the violence and announced that AU Commission President Jean Ping would be visiting Sudan within the next few days.

Relations between Sudan and Chad have been tense for some time as each accuses the other of being a safe haven for rebels. An EU peacekeeping force is deployed in eastern Chad, which borders with Darfur and has been most affected by the unrest and rebel attacks.

All schools in Khartuom were closed Sunday, which is a normal weekday in Muslim Sudan. Residents were urged to remain in their houses as far as possible even during the day.

In Brussels, the EU's top foreign-policy official Javier Solana condemned 'the attack in Khartoum (Omdurman) by elements of the Justice and Equality Movement', warning there could be no military solution to the crisis in Darfur

A statement issued by his office said Solana 'considers the latest events should provide the opportunity to trigger the strong impulse necessary to find a lasting solution to the crisis in Darfur.

'This goes through a sincere resumption of the political process, the effective deployment of the hybrid force on the ground and the genuine improvement of relations between Sudan and neighbouring Chad.'

The EU has its biggest-ever peacekeeping operation, some 2,400 men, in Chad and the Central African Republic, to keep the peace along Sudan's border.