KHARTOUM: After robberies and carjackings, Darfur "bandits" have turned to kidnapping foreign workers as a way to make easy money, promising to further hamper the already difficult job of humanitarian agencies in the war-torn region.

"They used to hijack convoys transporting petrol from Khartoum to Darfur, but now these convoys travel under military protection," Sudanese minister of state for humanitarian affairs Abdel Baqi Gilani said this week.

"They have therefore changed strategy and opted for something easy and simple -- kidnapping people and demanding a ransom."

Since the conflict began in Darfur in 2003, "bandits" have broken into the offices of aid groups and stolen equipment and money, and have carjacked hundreds of vehicles carrying aid or targeted peacekeepers themselves.

But no foreign aid worker had been kidnapped until an arrest warrant was issued in March by the International Criminal Court against Sudanese President Omar al-Beshir for alleged war crimes and crimes against humanity in Darfur.

In response, Khartoum expelled 13 international aid agencies -- accused among other things of spying -- and launched a campaign to "nationalise" humanitarian aid in Darfur, where 2.7 million people live in camps.

A week after the expulsions, members of the Belgian branch of Doctors without Borders (Medecins sans Frontieres, or MSF) were kidnapped in a north Darfur area controlled by Arab tribes.

Two aid workers from the French Aide Medicale Internationale were also abducted, bringing about the end of AMI operations in Darfur. All have since been released.

France in particular is not in Khartoum's good graces, analysts say, for its support for the ICC warrant, its hosting of Darfur rebel chief Abdel Wahid Nur and for its presence in neighbouring Chad.

Since the first two abductions, two employees of Irish aid agency GOAL, civilians with the UN-African Union joint peacekeeping mission (UNAMID) and an International Committee of the Red Cross staffer, a dual French-British national, have been kidnapped.

In the latest incident, two Sudanese staffers of Libya's Kadhafi Foundation were reported kidnapped in Darfur, although Sudan's state minister for humanitarian affairs told AFP on Friday the pair had never been abducted.

The two GOAL employees were released this week after 107 days in captivity.

"Arab tribes committed the abductions in Darfur," said a source in the peacekeeping mission, asking not to be named.

"Things do happen within the Arab tribes because they are angry with the government. But does this translate to kidnappings?" asked Fouad Hikmat of the International Crisis Group.

"You can't really just call them Arabs (in Darfur) because the word includes many realities. Are we talking about the Abbala, the Baqqara or Arabs that belong to the janjaweed?" militias affiliated to Khartoum.

Three theories have been circulating about the wave of abductions: the kidnappers want ransom money; they fought with the Sudanese army at the start of the conflict and now feel abandoned; or they are trying to empty Darfur of foreigners to "nationalise" aid work.

Regardless of theory, the result is the same. Hostage taking in Darfur has affected the work of aid agencies which have reduced their movements in remote areas and time spent on the ground by expatriate workers.