UN Secretary General Kofi Annan has called on the Sudanese government to allow a UN assessment team into the war-torn region of Darfur.
He said the international community needed to move rapidly "to have the right impact on the ground".
Top UN humanitarian official Jan Egeland has flown to Sudan and will visit Darfur on Sunday.
His visit comes a day after Khartoum and the largest rebel group in Darfur signed a peace deal.
Two smaller rebel groups rejected the deal, after intense talks in Nigeria.
The three-year conflict has killed about 200,000 and left about two million homeless.
Welcoming the peace deal, Mr Annan said the international community must immediately help strengthen the 7,000-strong African Union force in Darfur so it can implement the peace plan on the ground.
And he urged the Khartoum government to issue visas to his team of assessors so they can begin planning for the arrival of an international peacekeeping force to replace the African troops later in the year.
Khartoum has said in the past it would only consider inviting in UN troops if a peace deal was reached.
"Now is the time for them to allow the assessment mission to go in, for us to move expeditiously," Mr Annan said.
Mr Egeland, the UN's Emergency Relief Coordinator, will be the first UN official to visit Darfur since the signing of the peace agreement.
He said prior to his visit that access for aid workers in Darfur is at its worst level in two years.
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.
The signing ceremony was delayed for three hours, as mediators tried to persuade the smaller faction of the SLM to sign up to the deal.
But in the end it 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.
Mr Annan urged the two groups "to seize this historic moment and sign the agreement which will bring this tragic chapter in the history of Sudan to an end."
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.