Law and order has collapsed in the remote western region since mostly non-Arab rebels took up arms against Sudan's government in 2003, accusing it of neglecting the territory.

The two men and two women, all UNAMID police advisors, were last seen leaving their base near the capital of South Darfur Nyala on Sunday afternoon, Mezni said.

"They have been abducted. We are doing all we can to make sure they are released unharmed," the spokesman of the UNAMID force Noureddine Mezni told Reuters, adding it was not known what group carried out the kidnapping.

He added the force had so far not received any message from the abductors who also took the peacekeepers' vehicle.

Darfur has seen a wave of kidnappings of foreigners, including UNAMID officials, in the past year, mostly by armed young men demanding ransom.

Gunmen have now abducted at least 18 foreigners in Darfur and just over its border in neighboring Chad and the Central African Republic.

Humanitarian organizations working with more than 4.7 million people caught up in the conflict have reacted by pulling most of their expatriate staff back to Darfur's main cities.

But there have been signs the kidnappers have become more daring, seizing captives close to major settlements.

Two UNAMID civilian staff, a Nigerian man and a Zimbabwean woman, were captured in their compound in the west Darfur town of Zalingei in August and freed more than 100 days later.

The last foreign hostage in Darfur, Red Cross worker Gauthier Lefevre, was taken close to the capital of west Darfur El Geneina and released last month after 147 days in captivity.

Sudan's government has denied paying ransoms to resolve any of the kidnappings but aid groups fear at least rumours of large payments are fueling the trend.

All captives up until this week's incident have been released unharmed.

Mezni said the force had set up crisis management centers in Nyala and north Darfur capital El Fasher and was working with officials from the Sudanese government.

Aid groups in Darfur reported a surge of hostility to their workers after the International Criminal Court issued an arrest warrant for Sudan's president Omar Hassan Al-Bashir to face charges of war crimes in Darfur in March 2009.

Bashir accused humanitarian groups of passing information to the court, a charge they denied.

The kidnappings, previously almost unheard of in the region, started soon after the indictment.

UNAMID reported the four peacekeepers had gone missing earlier this week but at the time said it did not know what had happened to them.