Sudan's painful divorce has created more than half a million stateless orphans as a deadline passed for those the north considers southerners to leave or register as foreigners.
Hundreds of thousands of Sudanese, many of whom were born in cities such as Khartoum and have never visited the south, will now be considered "foreigners" in the Arab-dominated north after a transition period ended yesterday.
Nine months after South Sudan seceded to become the world's newest country, as many as 700,000 people have been stripped of their nationality because of family links to the south or religious affiliation.
The separation came after nearly half a century of civil war often pitting a predominantly African and Christian south against the majority Arab and Muslim north. However, the clean lines of separation suggested by a north-south confessional conflict have always jarred with the more complex mosaic of peoples and faiths on the ground.
For people like Osman, a 35-year-old engineer who lives in Khartoum who declined to give his second name, the undeveloped south is no more than a notional homeland.
"I am a victim of southern secession. I was born and raised in the north, have never been to the south and my wife is from the north," he told Associated Press. An Arabic speaker who doesn't share a language with the southern tribes or speak any English, he now faces a frightening limbo in which he risks separation from his family. "I want to stay in Sudan but the government does not allow it."
The southern exodus has been a "logistical nightmare" according to the International Organisation for Migration, who have assisted some 23,000 people to move, mainly by river. Huge barges have been arriving in the southern capital Juba almost daily for the past year, often carrying people who have left behind the wealthier and more developed north for a new country with few roads and almost no infrastructure.
The birth of a country in the south stranded millions of Sudanese on the wrong side of a contentious border. Tens of thousands of returnees who took the land route south have found themselves living in camps surviving on handouts. And the bitter conflict that led southerners to vote overwhelmingly for independence last year has left a legacy of mistrust and border disputes that threaten to provoke a new war.
A summit of the two countries' leaders scheduled for last week was cancelled after fresh clashes in the border area where the south has accused the north of bombing targets in the oil-rich Unity State.