Sudan's Abyei dispute: Minister Luka Biong Deng resigns
A southern minister in Sudan's national government has resigned, saying "war crimes" had been committed in the disputed Abyei region.
Luka Biong Deng said he could no longer work with the party of President Omar al-Bashir in the unity government.
He is a senior official in the south's ruling party, which is set to lead South Sudan to independence in July.
The two sides fought for decades before agreeing to share power and hold a referendum on southern independence.
Analysts fear the the dispute could reignite the north-south conflict, in which some 1.5 million were killed.
Northern troops seized the territory at the weekend after southern forces had ambushed a convoy of its forces in the area, killing 22 people.
Some 20,000 people have now fled the town, which has been left deserted, aid workers say.
UN human rights chief Navi Pillay said she had received reports that northern forces had been shelling and bombing civilian areas.
On Monday the UN said the town had been set ablaze, while gunmen were looting property following the northern takeover.
Mr Biong Deng is originally from Abyei, which both sides claim.
Its status was left undecided in the 2005 peace deal and a referendum, due in January, on whether the area should be part of the north or south has been postponed indefinitely.
"We had hoped that we could form two viable states in good relationship with each other, but those in Khartoum do not seem interested in peace," Mr Biong Deng said in his resignation statement.
"But with war crimes being committed in Abyei at the hands of the [President Bashir's] National Congress Party, I could not in good faith continue to take part in such a government."
His party, the Sudan People's Liberation Movement, has urged the northern troops to withdraw from the town.
The US envoy to Sudan, Princeton Lyman, has warned that the takeover could jeopardise possible debt relief worth billions of dollars.
He also said that Washington would find it difficult to remove Sudan from its list of state sponsors of terrorism unless troops were withdrawn from Abyei.
Under these sanctions Sudanese companies are banned from using US currency - a major obstacle to international trade.
The US has previously suggested that a peaceful transition to independence for the south and a negotiated solution to the separate conflict in Darfur could normalise relations.
Both Sudan and the South are reliant on their oil revenues, which account for 98% of South Sudan's budget. But the two countries cannot agree how to divide the oil wealth of the former united state. Some 75% of the oil lies in the South but all the pipelines run north. It is feared that disputes over oil could lead the two neighbours to return to war.
Although they were united for many years, the two Sudans were always very different. The great divide is visible even from space, as this Nasa satellite image shows. The northern states are a blanket of desert, broken only by the fertile Nile corridor. South Sudan is covered by green swathes of grassland, swamps and tropical forest.
Sudan's arid north is mainly home to Arabic-speaking Muslims. But in South Sudan there is no dominant culture. The Dinkas and the Nuers are the largest of more than 200 ethnic groups, each with its own languages and traditional beliefs, alongside Christianity and Islam.
The health inequalities in Sudan are illustrated by infant mortality rates. In South Sudan, one in 10 children die before their first birthday. Whereas in the more developed northern states, such as Gezira and White Nile, half of those children would be expected to survive.
The gulf in water resources between north and south is stark. In Khartoum, River Nile, and Gezira states, two-thirds of people have access to piped drinking water and pit latrines. In the south, boreholes and unprotected wells are the main drinking sources. More than 80% of southerners have no toilet facilities whatsoever.
Throughout the two Sudans, access to primary school education is strongly linked to household earnings. In the poorest parts of the south, less than 1% of children finish primary school. Whereas in the wealthier north, up to 50% of children complete primary level education.
Conflict and poverty are the main causes of food insecurity in both countries. In Sudan, many of the residents of war-affected Darfur and the border states of Blue Nile and South Kordofan, depend on food aid. The UN said about 2.8m people in South Sudan would require food aid in 2013. The northern states tend to be wealthier, more urbanised and less reliant on agriculture.