Stephane Dujarric, a UN spokesman, announced on Friday that the world body would move 105 personnel, mainly military experts, to Darfur in the next few weeks to man communications and help with transport in anticipation of an eventual transition to a UN force.
But the package of assistance does not have Sudan's permission yet, a senior UN official said, speaking on condition of anonymity.
Kofi Annan, the UN secretary-general, and Alpha Oumar Konare, chairman of the AU Commission, have written to Omar Hassan al-Bashir, the Sudanese president, "to inform him" of the technical support to African troops, Dujarric said on Tuesday.
The AU force of some 7,000 troops and monitors has agreed to stay until the end of the year to help stop atrocities in Sudan's western region. It has asked for help from the UN and for funds from the Arab League and promised to raise another 4,000 troops.
But the AU force has been unable to stop the violence in Darfur that has driven 2.5 million people from their homes and left an estimated 200,000 dead since 2003.
Short of equipment, funds and soldiers, the AU is in favour of turning over control of the peacekeepers to the United Nations but Bashir has refused, saying it would only divide and weaken Sudan.
Still, the UN on Monday called together 49 potential troop contributors for its own force and received some pledges for infantry soldiers from Tanzania, Nigeria and Bangladesh.
Espen Bartheide, Norway's state secretary for defence, said his country, together with Sweden, had offered a joint battalion of about 500 engineers and other experts for the still non-existent UN force and authorised 20,500 military personnel and police.
Bartheide said the new UN force should be dominated by African troops with other countries providing "quality equipment".
But he said that except for three Norwegian logistics experts, the engineering battalion would not be part of an AU force but would wait until the UN took charge.
Some diplomats believe countries should beef up the AU operation rather than push for UN control.
But Bartheide and UN peacekeeping officials say the only viable operation is a transition from the AU to the UN and not an "AU plus".