Top UN humanitarian official Jan Egeland has arrived in Sudan to review the situation in the Darfur region.
Hundreds of thousands have been displaced by the violence
His five-day visit comes amid signs the government may allow a UN peacekeeping force into the war-torn area.
On Friday Khartoum and the largest rebel group in Darfur signed a peace deal. Two smaller rebel groups rejected the agreement, after talks in Nigeria.
The three-year conflict has killed about 200,000 people and left about two million homeless.
UN Emergency Relief Co-ordinator Mr Egeland is due to go to Darfur on Sunday, in the first visit by a UN official to the region since the peace deal was signed.
He is expected to visit southern Darfur, where fighting has broken out recently.
Mr Egeland will have talks with local leaders and visit refugee camps before heading to Khartoum on Monday for meetings with Sudanese officials, according to French news agency AFP.
He said prior to his visit that access for aid workers in Darfur was at its worst level in two years.
The trip came amid hopes Khartoum might accept a UN force to take over from African Union (AU) troops in Darfur.
Khartoum has said in the past it would only consider inviting in UN troops if a peace deal was reached.
But on Saturday the US welcomed Khartoum's "new willingness".
Citing comments by government representatives indicating they were now willing to accept the UN force, US Ambassador to the UN John Bolton said they viewed "this as... the first positive outcome from the Abuja peace agreement", according to the AFP news agency.
Earlier UN Secretary General Kofi Annan urged Khartoum to issue visas to his team of assessors so they could begin planning for the arrival of an international peacekeeping force to replace the 7,000 African troops later in the year.
The peace plan, brokered by the African Union, creates a temporary regional government for Darfur, in which rebels will take part.
The pro-government Janjaweed militia are to be disbanded and the rebels incorporated into the security forces.
Deadlines came and went in recent days, as diplomats exerted pressure on parties after all the rebels had rejected the original draft.
In the end the Khartoum government and the Sudanese Liberation Movement (SLM) said they were willing to sign, despite reservations on both sides over power sharing and security.
But the smaller faction of the SLM would not budge, blaming a lack of trust in the security arrangements.
The smallest rebel group, the Justice and Equality Movement (Jem), called for fundamental changes to the document.
Its chief negotiator reiterated the rebels' demands for the post of vice-president in the Khartoum government and for Darfur to have a greater share of national wealth.
Aid organisations say the conflict has created one of the worst humanitarian crises in the world.
The rebels took up arms in 2003, accusing the government of discriminating against the black African residents of Darfur.
Pro-government Arab militia then launched a campaign, described as "genocide" by the US.
The Sudan government denies backing the Janjaweed militias accused of mass killing, rape and looting.