"The three World Food Programme staff members and three members of Sudanese Red Crescent were released by the SLM this morning," Peter Smerdon told AFP by phone on Wednesday.
There were only three Sudanese Red Crescent (SRC) workers, and not five as earlier thought when they were snatched on Saturday, he explained.
"They were flown by helicopter to al-Fasher after being picked in the Tabit area, south of al-Fasher," he said, adding that "they are well and unharmed".
On Tuesday, 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 SRC 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.