Speaking from Darfur, Jan Egeland told the BBC there was a sea of militias in the region and without security the humanitarian work could not succeed.
Mr Egeland is in Darfur, in western Sudan, to assess conditions there and in refugee camps in neighbouring Chad.
He is to hold talks with the government in the capital, Khartoum, on Monday.
The government and the biggest rebel group signed a peace deal on Friday.
The agreement has raised hopes the government will allow UN peacekeepers to take over from African Union troops in the region. It has previously said it would only consider doing so if a peace deal was signed.
Some 7,000 African Union troops are in the region - an area the size of France - but have struggled to contain the fighting.
DARFUR PEACE PLAN Pro-government Janjaweed militia to be disarmed Rebel fighters to be incorporated into army One-off $300m transfer to Darfur $200m a year for the region thereafter Compensation for those forced to flee their homes Regional government, if approved in a vote
Q&A;: Darfur crisis Have Your Say: Darfur crisis
"Half of the population now has become war victims...so I believe, yes, we are turning the corner, but the whole world has to put pressure on the parties."
He was speaking in the town of Gereida which is held by the rebel group which signed the peace deal, the Sudan Liberation Movement under Minni Minnawi.
Just a month ago, the government banned Mr Egeland from visiting Darfur.
Tens of thousands of people around Gereida have been displaced in recent weeks, and the UN has warned of an impending disaster.
Earlier, Mr Egeland held talks with the deputy governor of South Darfur in the town of Nyala and called for aid workers to be given unlimited access, as agreed in the peace deal.
On his arrival at Nyala airport he was greeted by a small government-backed anti-UN demonstration.
The peace plan, brokered by the African Union, creates a temporary regional government for Darfur, in which rebels will take part.
But two smaller rebel groups rejected the agreement.
The three-year conflict has killed about 200,000 people and left about two million homeless.
Aid problem
Aid organisations say the conflict has created one of the world's worst humanitarian crises.
Money for the world's largest aid operation is running out. Rations for May have been cut in half and many of Darfur's 2m displaced people will go hungry.
The BBC's Jonah Fisher says Khartoum appears to be sending out mixed messages on the issue of whether to allow UN peacekeepers into the region.
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.
The Government of Sudan has expressed that the  Darfur Peace Accord with main rebel group, Sudan Liberation Army, led by Minni Menawi, can overturn the previous rejection of UN Peacekeepers. So far, only African Union Peacekeepers were allowed in Darfur.

"The Sudan government will be open for any assistance," Bakri Mulah, secretary-general for external affairs in Sudan's Information Ministry, said in Khartoum, Sudan.

In New York, John R. Bolton, the US ambassador, on Saturday welcomed Khartoum's new willingness to accept a UN peacekeeping force to take over from AU soldiers in the Darfur region. He cited the Sudanese government representatives' comments  to indicate they were now willing to accept the UN force. He said, " We view this as a very encouraging sign, the first positive outcome from the Abuja peace agreement. "

In the meantime, top UN humanitarian official Jan Egeland has arrived in Sudan to review the situation in the Darfur region. He is expected to visit southern Darfur, where fighting has broken out recently.  It is the first visit by a UN official to the region since the peace deal was signed.

He said that access for aid workers in Darfur was at its worst level in two years. "At the moment Darfur is slowly being strangled, it's dying in front of us, half of the population now has become war victims...so I believe, yes, we are turning the corner, but the whole world has to put pressure on the parties." said Mr Egeland, the UN's Emergency Relief Co-ordinator.  

Earlier, UN Secretary General Kofi Annan requested Khartoum to issue visas to his team of assessors so they could plan for the deployment of an international peacekeeping force to replace the 7,000 African Union troops later in the year. The AU troops, now running out of funds, had largely been ineffective in stopping atrocities, leaving millions to struggle in camps with little food or water. While details for a UN peacekeeping force are finalised, the US diplomat said the US had asked Rwanda to send in some 1,200 troops to supplement the AU forces.

In a recent development, an unknown assailant shot a Spanish United Nations Aid worker in Chad. She is working in the eastern part of Chad to assist the refugees from Darfur region. 2,00,000 refugees have crossed the border to escape from the violence in Darfur. She is in critical condition.