Thousands of southern Sudanese are fleeing the north as tension grows in the build-up to January's referendum on possible southern independence.
Southern Sudan's government is trying to organise many of the returns.
Many southerners have been scared by suggestions from senior northern officials that they would not be welcome if the south votes to secede.
The referendum is part of a 2005 deal to end the 21-year civil war in Africa's largest nation.
The Muslim north and south where most people are Christian or follow traditional religions are also divided along ethnic, economic and political lines and have fought for most of Sudan's post-independence history.
'Uncertain in south'
The government of the south is paying for transport for many of those wanting to leave the capital, Khartoum - although some people have been waiting for weeks for a bus.
"I am going to the south because the referendum is near," Michael Mashot told the BBC.
A thin man carrying a luminous green plastic chair on his shoulder, he says he is returning to Bentiu, the capital of Unity State.
"It's difficult to be a southerner in Khartoum because this place is not my place."
The south's Unity State has set up a makeshift office in the Khartoum suburb of Sahafa, and has already registered more than 5,000 people to get buses heading south - a journey which can take several days.
Men mill around outside the office, while women sit on mats on the dusty street, clutching small children on their laps.
Mattresses and chairs and tightly stuffed plastic bags of household belongings are piled up on a scrap of wasteland.
UN head of humanitarian affairs Baroness Valerie Amos, who has been touring Sudan this week, is worried about too many people returning to the south, to uncertain conditions.
"Where people want to return, it's important this return is done on a phased basis," she says.
"There is no point people leaving jobs in the north to go to the south to be unemployed."
David Ring Gai, a young man who fled Sudan's civil war and lived for several years in a refugee camp in Kenya, moved to Khartoum to study.
Now he is preparing to leave - with the intention of voting a new country into being.
"Many people are gathering to go back to their homelands to vote for separation, not for unity," he explains.
"We are here for many years suffering from the Khartoum regime."
He says he is worried he would not get a fair vote if he stays in the north.
"Whenever we vote here we cannot get good voting. They can put that we are voting for unity but we are voting for separation."
The Sudan People's Liberation Movement, which controls the south since a 2005 peace deal, clearly agrees.
It is trying to move as many southerners as possible to vote in areas away from northern control - preferring them to be in places it influences.
At one point it announced it wanted more than a million southerners to return, though this number has been revised down.
But outside the Unity State office in Sahafa, frustration is growing as there is no money for the buses to transport people back home.
"I've been waiting here for almost three months," Mr Gai says.
"We are waiting for southerners to bring money to transfer the people to the south.
"There are many children; they sleep on the road without any shelter and without any food.
"We are suffering too much."
Simon, a young man with a large watch almost slipping off his slim wrist, says he too is desperate to leave Khartoum.
"We are disturbed; we don't know what is going on. We just want to get to Southern Sudan."