She said that the attack on a joint military and police patrol lasted two hours and the assailants used heavy weapons.
Suna, Sudan's official news agency, reported that the attackers were travelling in a convoy of about 40 vehicles.
The 9,000-strong United Nations-AU force has been in place in Darfur since earlier this year.
It is authorised to have 26,000 members, but has suffered from shortages of staff and equipment and a lack of co-operation from the Sudanese government.
Unamid says that the US and other governments have not supplied the helicopters, surveillance aircraft, military engineers and logistical support needed for it to operate effectively in the region.
Last month, four members of the force were assaulted and held at gunpoint in Darfur, with one staffer stripped of his belongings, according to a Unamid statement.