Threats of violence forced the United Nations to evacuate large areas of war-torn Darfur yesterday, underlining Sudan's failure to curb the notorious janjaweed militia.

UN aid workers helping desperate refugees were withdrawn from outlying areas of Western Darfur province and brought to the safety of the local capital, El Geneina.

Gunmen from the janjaweed, which translates as "demons on horseback", have issued new threats to aid workers and food convoys.

The janjaweed, who have mounted a brutal campaign against Darfur's black African tribes, were the subject of a special UN resolution passed last July.

The mandatory resolution demanded that the janjaweed be brought under control by last Sept 1. Yet more than six months later their threats have obliged the UN to remove its staff.

"The janjaweed militia have said that they will now target all foreigners and all UN humanitarian convoys, so we have withdrawn all people to El Geneina," said Jan Pronk, the UN's envoy in Sudan.

Some 44 staff have been moved so far and will now stay in El Geneina, the only area of Western Darfur considered safe, until further notice.

The UN and other agencies took the drastic action after janjaweed gunmen waylaid UN lorries delivering aid to Darfur's refugee camps. Their Sudanese drivers were told that all foreigners would now be targeted.

Mr Pronk suggested that the renewed threat might be linked to an order from the provincial governor for the janjaweed to return vehicles given to them by the regime last year.

The Khartoum government has always denied supporting the janjaweed and President Omar al-Bashir has repeatedly called them "bandits".

But a UN investigation concluded that the regime had armed and supplied the janjaweed in order to carry out a brutal counter-insurgency campaign, involving countless attacks on the black African tribes accused of backing Darfur's rebels.

In two years of fighting, about two million people have been forced to flee their villages for squalid refugee camps, with 200,000 pouring over the border into neighbouring Chad.

War-induced starvation and disease have killed at least 180,000 people, with tens of thousands more dying at the hands of rebels from the Sudan Liberation Army, the janjaweed, or government forces.

All sides have broken a ceasefire agreed last April. A second UN resolution passed last September held out the prospect of imposing oil sanctions on Sudan unless the violence ended. But China has large interests in Sudan's oil industry, which produces 345,000 barrels every day, and has pledged to block any embargo.

The only outside forces present in Darfur are about 3,000 African troops protecting military observers from the African Union, an alliance of all 53 countries on the continent. They have neither the training nor the numbers nor the mandate to protect civilians.

The latest fighting in Western Darfur claimed about nine lives on Wednesday. Rebels accused government forces of attacking two villages in the Jabel Moun area.

The African Union's envoy to Sudan cited these latest clashes as evidence that security in Darfur is worsening.

Baba Gana Kingibe said he was "disappointed" by the renewed fighting, adding: "Unfortunately in the last one week the situation has deteriorated."