Pretoria, South Africa (CNN) -- Half a million people will face an emergency bordering on famine by March if international humanitarian organizations are not allowed into areas of Sudan that are mired in conflict, United States envoy to Sudan Princeton Lyman warned Wednesday.
"There is a looming humanitarian disaster in Sudan," Lyman warned in the South African capital Pretoria as part of his effort to rally support for African intervention in Sudan.
Groups like the World Food Programme and UNICEF must be able to work in the border areas of South Kordofan and Blue Nile, he said.
The government in Khartoum argues that these areas are too dangerous for NGOs to operate in.
Lyman urged South Africa to show leadership on the issue.
"As chair of the United Nations Security Council this month, South Africa can play a role in preventing a colossal disaster," Lyman said.
The American envoy said a lack of leadership, a history of vicious ethnic violence and the indictment of Sudan's President Omar al-Bashir by the International Criminal Court are all factors that have complicated the crisis in that country.
"It falls to national leaders to make peace and to make decisions that could lead to peace," Lyman said.
He lamented the lack of a mechanism for Sudan and newly independent South Sudan to resolve conflicts, "despite the two sides having been in a unity government for five years and having participated in years of peace talks."
Since South Sudan gained independence from the north in July 2011, fighting has continued in the oil-rich border regions of South Kordofan, Blue Nile, Unity State and Upper Nile.
Countless lives have been lost and hundreds of thousands have been displaced, exacerbating an already desperate humanitarian crisis, according to rescue officials.
The two sides continue to bicker over oil revenue, and the South has also been plagued by deadly tribal wars.
"Africa needs to speak with one voice," Lyman said. "Africa needs to say, 'We cannot allow this to happen.'"
The Sudanese ambassador to South Africa, Ali Yusuf Alsharif, warned that the situation in his country could become worse than Somalia -- but he said that outside pressure will further complicate the situation.
"The world has looked at Somalia, not knowing what to do," he said at the conference. "But if you push everyone (in Sudan), you could have a situation worse than Somalia."