Jan Pronk said on Wednesday: "While the (Sudanese) government has shown its concern with the task of the team, the other parties have not yet shown the required response."
"The UN is presently exercising pressures on the armed movements to consent to the AU team's assignment of locating the position," he said.
The UN envoy stressed that cooperation with the AU mission was an obligation on both sides under UN Security Council resolutions.
The Sudan Liberation Army and Justice and Equality Movement had "refused to cooperate with the AU verification team," junior foreign minister Al-Tigani Salih Fidhail said.
Khartoum "awaits a statement by the AU on the matter and, at the same time, awaits the international community and the European Union to declare their positions with regards to the attitude of the rebels towards the verification team," he said, stressing his government's "full cooperation".
Aljazeera has obtained a copy of a statement issued by a Sudanese rebel movement announcing it had joined forces with rebels in the east.
The rebels say they will start attacking a military camp north of Kassala, near the eastern border.
"While the (Sudanese) government has shown its concern with the task of the team, the other parties have not yet shown the required response"
UN special envoy
One of the two main Darfur rebel groups, the Justice and Equality Movement (JEM) made the announcement stating that this is the beginning of an armed struggle to overthrow the government in Khartoum.
Also on Wednesday, the coalition of rebels captured three ruling party politicians as they returned from a conference aimed at preventing conflict in Sudan's east, a government official says.
Three politicians seized
The three men were seized after leaving the government-organised conference in the town of Kassala near the Eritrean border on Tuesday.
"These three left early at about 11am (0800 GMT) so they were not in the armed convoy which left after the conference ended at about 4pm," Wasiyla said.
"The rebels wanted to kidnap many people from the conference to damage its good work, but the last session finished late so they only managed to kidnap these three members of the state assembly," said the Red Sea state's governor, Hatim al-Wasiyla.
The convoy, transporting the other delegates, arrived safely in Port Sudan from Kassala.
The three politicians were from the ruling National Congress Party that organised the two-day conference.
No demands yet
Wasiyla said local tribal leaders were in contact with the rebels, who said the three were in good health. But, he said, the rebels had not yet made any demands for their release.
The eastern rebel groups, the Free Lions and the Beja Congress, occasionally clash with government forces.
"The rebels wanted to kidnap many people from the conference to damage its good work..."
Red Sea state governor
The Darfur rebels took up arms more than two years ago in the west in fighting that has killed tens of thousands and forced more than 2 million from their homes.
Both sides complain of marginalisation by the government, accusing it of concentrating on the country's central region.