"I call on the international community to be strongly involved in the recovery of Sudan," Guterres told reporters on Wednesday in the Kenyan capital, Nairobi, following a 10-day visit to Sudan, Chad and Kenya that focused on the Sudanese refugee situation.
He said the UN refugee agency, UNHCR, was doing as much as it could - with limited funds - to improve life in southern Sudan so people who fled their homes during the 21 years of civil war between the north and south could go home.
"During centuries, very little has been done in southern Sudan," he said. "We have not a problem of reconstruction; we effectively have a problem of construction. Most things are missing."
As refugees and internally displaced people (IDPs) within Sudan began returning home, he said, UNHCR was drilling boreholes and building schools and health clinics in the south to improve conditions for people to return, rebuild their lives and remain in their homeland for good.
Although UNHCR was not a development agency, Guterres noted, it had a responsibility to make sure certain services were in place when the refugees arrived home.
"It is our mandate to support refugees, and facilitating their return is part of it," he said.
"We can help with some schools, some health centres, but we have not the money to help with everything you need," he said on Sunday at a primary school being built by UNHCR in Yari, southern Sudan - the only one to serve a community of some 28,000 people.
The High Commissioner drew a clear link between development aid, economic growth and peace for a region where even today, landmines deface the landscape and claim lives.
"If we want Ugandans to be in Uganda, Sudanese in Sudan and Portuguese in Portugal, we must stop war," the former Portuguese prime minister said. "But it is very difficult to have peace if everybody is poor, if people don't have enough to eat, if children don't have schools."
UNHCR, he said, was trying to attract more traditional development agencies to create conditions for prosperity to ensure that returnees did not have to flee again.
"Sudan is not only the country with the greatest displacement, but it presents a very relevant country for the progress, development and stability of the region," he said.
He added: "We hope that after the rainy season [in October] we will be able to start a movement of coordinated return - from Congo, from the Central African Republic, hopefully also from Kenya and Ethiopia - to southern Sudan."
Apart from Afghanistan, he noted, there were few operations in the past that had been bigger than this one.
There are an estimated 613,000 southern Sudanese refugees outside the country and some 4.5 million displaced within Sudan. In addition, some 200,000 refugees and 1.8 million displaced people have fled the western Sudanese region of Darfur in the last two years.
On the conflict in Darfur, the High Commissioner said he saw a window of opportunity to reach a political settlement for the strife-torn region. He called upon the international community to put pressure on the government of Sudan and the rebel movements to reach an agreement.
"Insecurity is still there. If peace is not reached, I look with a lot of anxiety to what might happen," Guterres warned. He noted that a large part of Darfur's population was still living in camps and many people were carrying arms.
"The problem is basically a security and political problem, so it can never be solved with a humanitarian solution alone," he added.