Mbeki, the former South African president, is expected to spend two days in Sudan before visiting South Sudan, to press both sides to return to talks to resolve their disputes over oil transit fees, their border and other issues.
The peace effort comes with the U.N. and aid agencies warning of a growing humanitarian catastrophe due to the conflict. The U.N. said this week that half South Sudan's population faced possible hunger this year.
Early this month, after the two countries lurched dangerously toward war, the Security Council gave them two weeks to resume peace talks and threatened them with sanctions if they failed to take steps to settle their differences.
South Sudan seceded from Sudan in July, and though Khartoum agreed to its independence, relations have continued to deteriorate in disputes left unresolved in a 2005 peace agreement, which ended 22 years of civil war.
South Sudan turned off oil production in February after Sudan seized several shipments of southern oil in a row over oil transit fees, and relations have soured further since. The situation reached a flash point when South Sudan took control of Sudan's most important oil-producing area, Heglig, on April 10, occupying it for 10 days before withdrawing.
Both countries are dependent on oil and face a growing economic crisis without oil revenue. Both are seeing sharply rising prices for food and fuel and collapsing currencies.
Johnnie Carson, U.S. assistant secretary of State on Africa, on Wednesday called on both sides to return to the negotiating table.
"Let me say that both sides bear a great amount of responsibility for creating the difficulties that exist today," Carson said in a conference call with African journalists. "It is incumbent on both parties to seriously negotiate their issues. They have to come to the table and meet on a regular basis and live up to the commitments that they make.
"They must sit down and seriously negotiate their differences and recognize that they cannot make progress on the battlefield."
Carson said Mbeki was encouraging both sides to put proposals on the table regarding their key disputes.
"While there has not been a full resumption of discussions between the two sides, things are being done," Carson said. "We want them to be done much faster, and with greater alacrity and commitment."
The U.N. Office for the Coordination of Humanitarian Affairs warned in a report Tuesday that South Sudan could face destitution less than a year after its birth.
The office said the oil shutdown and budget austerity measures could push millions of South Sudanese into destitution, as fuel and food prices rose sharply. Adding to the pressure were the 130,000 refugees in South Sudan from Sudan and other neighboring countries.
The agency said about 4.7 million people -– well over half of the population -- were at risk of hunger this year, including at least a million who faced severe food shortages. The remaining 3.7 million would need food aid to avoid slipping into severe crisis.
Oxfam and four other humanitarian agencies warned Tuesday that with the rainy season beginning, the humanitarian refugee crisis could soon become a catastrophe.
“A toxic combination of conflict, rising food and fuel prices, and severe cash shortages is having a devastating effect on the civilian population in both countries. With the rains on the way the situation could not be more critical. We urgently need the fighting to stop so that we can get access and children can be protected from violence, deprivation, displacement and recruitment," said Jon Cuncliffe, Save the Children country director for South Sudan in a joint statement with other humanitarian agency officials.
"After more than 10 months of fighting, with no sign of peace, we're on the path from crisis to catastrophe," said Johnson Byamukama, Oxfam's deputy country director for South Sudan. "The coming rains could make life for refugees unbearable and bring the threat of waterborne disease. The world needs to wake up to the true cost of conflict for people who have already suffered so many years of war."
-- Robyn Dixon
Photo: African Union mediator and former South African President Thabo Mbeki, center, arrives at Khartoum airport May 17, 2012. Credit: AFP/Getty Images