Sudan's air force killed or wounded about 100 people and forced thousands to flee when it bombed a town in western Sudan this week, the UN has said.

UN spokeswoman Radhia Achouri quoted the African Union (AU) as saying Sudanese forces had bombed the Darfur town of Shangil Tobaya on Wednesday, but that it had not given a specific death toll.

The fighting forced the UN to withdraw staff from the area. "Whenever we have incidents like that we withdraw our people," said Achouri.

The AU is monitoring a shaky ceasefire signed in April between the Sudanese government and rebels in the Darfur province.

SPLA opens office

Achouri's comments came as rebels of the Sudan People's Liberation Movement (SPLM) opened offices in the capital, Khartoum.

John Garang's SPLA has become a political force in Sudan

"This is an historic moment," SPLM executive committee member Paul Thomas said on Friday at the inauguration ceremony. Hundreds of people, drawn from opposition parties as well as supporters of the rebels, attended the ceremony.

"Instead of only hearing about it from a distance, people are now seeing the SPLM physically among them," said Thomas, to shouts of: "Long live the SPLM. Long live freedom," from the crowd.

The offices used to house US financial services giant Citibank before Washington imposed economic sanctions against Khartoum in the mid-1990s.

To mark the change of use, the rebels made the ritual sacrifice of a bull, and onlookers described it as a "sign of peace and victory".