UN officials have said that the expanded Darfur force will consist of between 17,500 and 19,600 troops, in addition to more than 6,000 police who will help maintain security in the region's displaced person camps where allegations of rape and other abuses remain widespread.
In the face of intense international pressure and the threat of tougher UN sanctions, Khartoum gave approval "in principle" to the proposed new force.
The Sudanese government had previously endorsed only logistical support from the UN for the existing 7,000-strong AU force, which has struggled to patrol a region the size of France.
Ban Ki-moon, the UN secretary general, hailed Khartoum's "in principle" agreement as "a milestone development".
Washington, which accuses the Arab-led government in Khartoum of genocide in Darfur, voiced scepticism over the government's decision, and said it still intended to impose new sanctions.
On Wednesday, George Bush, the US president, said the US was "tightening existing economic sanctions against Sudan, and we're imposing additional ones," on Wednesday.
The African Union has acknowledged that a great deal of work still needs to be done before the additional troops and police are in place in war-torn Darfur.
On Saturday, Said Djinnit, head of the AU peace and security council, said: "We have achieved a great breakthrough, next is the implementation. It has to start. A number of things have to be put in place."
The UN estimates that at least 200,000 people have died and more than two million people have fled their homes since the rebellion erupted in Darfur in February 2003, drawing a scorched earth response from the Sudanese army and its Arab militia allies.
Other sources give higher estimates although the Sudanese government disputes the figures.
Kumalo's statement came during a press conference held by a delegation comprising of representatives of the member states in the UN Security Council at the end of a one-day visit to Sudan.