The head of Sudan's vaccination programme and emergency operations office, Hasan Idris, said rebels from the Justice and Equality Movement (JEM) abducted the volunteers on Sunday.
"We have no information regarding where these volunteers have been taken," he said on Monday, adding they had been working in Adwa, in the Shiariya area of Southern Darfur state.
JEM officials were not immediately available to comment.
Sudan said on Monday rebels had kidnapped eight World Food Programme (WFP) and Sudanese Red Crescent workers in Northern Darfur state. The WFP confirmed those workers were missing and set up a crisis centre to try to find them.
The United Nations' top envoy in Sudan, Jan Pronk, is to address the Security Council on 2 September on the Khartoum government's compliance with UN resolution to provide more security and aid access in Darfur.
After years of low-intensity conflict between nomads and farmers over scarce resources in arid Darfur, rebels revolted last year, accusing the Sudanese government of arming "militias" known as Janjawid to loot and burn homes.
Attacks on civilians
Khartoum admits arming some fighters to combat the rebels, but denies any link to the Janjawid, calling them outlaws.
They also accuse the rebels of starting the conflict and committing atrocities.
More than a million people have fled their homes, with about 200,000 refugees now in neighbouring Chad, triggering what the United Nations calls the world's worst humanitarian crisis.
International officials say the violence in the region is far from over.
Obasanjo says Khartoum has
launched attacks on civilians
Nigerian President Olusegun Obasanjo, who is currently chairman of the African Union, said on Monday the AU monitors had confirmed accusations by Darfur rebels that Khartoum launched fresh attacks on civilians last week.
Obasanjo said he had written to Sudanese President Umar Hasan al-Bashir asking him to ensure all attacks on civilians by government forces and Janjawid stopped, to avoid undermining peace talks being held in the Nigerian capital Abuja.