KHARTOUM (AFP) - Sudan should not strip ethnic southerners of their Sudanese citizenship if they are unable or unwilling to become citizens of newly independent South Sudan, Human Rights Watch said on Saturday.
The South separated last July following an overwhelming vote in the wake of a 2005 peace accord ending 22 years of civil war which killed two million people, and drove many more to the north.
An estimated 500,000 South Sudanese remain in Sudan, and Khartoum has given them until April 8 to leave or regularise their status.
"Many fled the long civil war in the south and have lived in Sudan for decades, or were born there and have few ties to South Sudan," the New York-based HRW said in a statement.
It said that under South Sudanese law any southern ethnic group member, or people with ancestors born in the south, are eligible for nationality.
Sudanese officials are interpreting these laws to mean that anyone with even one great grandparent born in the South will lose Sudanese nationality, "irrespective of whether they have acquired -- or want to acquire -- South Sudanese citizenship," HRW said.
"There are signs that Sudanese authorities have already begun to strip people of their citizenship, in violation of international law," it said.
On a visit to South Sudan earlier this year, the head of the United Nations refugee agency, Antonio Guterres, said he believed most southerners still in the north wanted to go South.
The International Organisation for Migration (IOM), which is dependent on donor funding, has helped to move more than 24,000 southerners from Sudan to South Sudan, mostly by river barge and train.
It says it is logistically impossible to transport half a million people by the April 8 deadline.
Those wishing to stay in the north are caught in a vicious circle because they need southern documents in order to apply to stay in Sudan, yet the South "has no appetite for documenting people in the north," Jill Helke, head of the IOM Khartoum office, told AFP earlier this year.
At the same time, even those who have obtained a South Sudanese passport and returned north are not able to legalise their stay "because the government of the north has not created the mechanism or decided how they're going to deal with it", she said.
Human Rights Watch said those wishing to remain Sudanese should be allowed to, while South Sudan should ensure that people wanting to become its citizens have access to needed documentation.
During a visit to Khartoum on Thursday, Britain's Under-Secretary of State for International Development, Stephen O'Brien, encouraged Sudan to extend the April 8 deadline.