Jan Egeland, the UN under-secretary for humanitarian affairs on his first visit to Darfur since Sudan barred him from visiting last month, said he would press rebel leaders outside the peace agreement to join it.
Sudan's government and the largest rebel group in Darfur, a faction of the Sudan Liberation Army (SLA), signed the agreement in Abuja, Nigeria, on Friday, but two other factions have refused to do so.
Aid workers in Darfur say they face the threat of expulsion and are hindered from providing aid by government restrictions on movement.
The peace agreement calls on all parties to improve humanitarian access to help aid operations.
"In the peace agreement in Abuja, there is unlimited access granted in all Darfur for all humanitarian organisations, but this is not the practice," Egeland said after meeting Salah Mostafa, a deputy governor for South Darfur state.
"We need more resources for humanitarian work from the whole world, including the West, Islamic and Arab countries and Asian countries," he told reporters.
The United Nations said last month that it would cut food rations for more than 6 million people in Sudan, half of them in Darfur, due to a lack of funds.
Egeland talks to a tribal leader
in the town of Gereida
In Nyala, the capital of South Darfur state, a group of about 25 men protested against Egeland's visit.
A banner read: "We condemn the aggression by Scandinavian countries against the Prophet Mohammad", a reference to cartoons in a Danish newspaper of the Prophet Mohammad.
Egeland is Norwegian, and Sudan said one of the reasons he could not travel to Darfur in April was because it could not guarantee his safety.
Egeland later travelled to the rebel-held area of Gereida, where he urged SLA commanders to instruct their forces to respect the agreement.
"Never before has so much been at stake in Darfur. This is the moment of truth not only for the parties to show their willingness to help their own people to get out of this vicious circle, but also for the international community to show solidarity," he told Reuters.
Egeland said the African Union force in Darfur had to monitor whether the peace deal was respected.
Representatives for Sudan and the
SLA sign the peace deal in Abuja
Three years of fighting between Darfur rebels and the Sudanese government has killed tens of thousands of people and forced 2 million to flee their homes.
On Sunday, a human rights group said Darfur's warring factions must stop attacking humanitarian agencies.
Human Rights Watch, in a briefing, said aid workers are struggling to reach civilians in dire need of help.
"After three years of horrendous attacks, more than three million people in Darfur depend on international aid to survive," Peter Takirambudde, Africa director at Human Rights Watch, said in a statement.
"The warring parties must give humanitarian aid workers access to those in need," Takirambudde said.
The group said civilians are at risk from attacks in Darfur as aid workers face mounting insecurity, targeted attacks and "administrative obstruction" by the Sudanese government.
Under international humanitarian law, the group said the warring parties "must allow impartial humanitarian agencies to assist civilians, and attacks on aid workers or deliberate obstruction of relief efforts can constitute war crimes".