South Sudanese 'press-ganged' by rebels in Khartoum
Young South Sudanese men living in Sudan's capital, Khartoum, are being forcibly conscripted by militia groups, numerous sources have told the BBC.
It is alleged they are forced to fight for rebels in South Sudan, which split from the north in July.
South Sudan's information minister believes Khartoum is directing the rebel groups and the kidnappings.
A senior official in Sudanese President Omar al-Bashir's party told the BBC the accusation was "nonsense".
It is feared the alleged abductions will worsen the already fragile relationship between the two countries.
According to South Sudanese community leaders, church workers in Khartoum and politicians in South Sudan, men have been snatched from universities, the streets and even their homes by armed gangs.
"The attitude of recruiting South Sudanese university students into the military by the Khartoum regime is an irresponsible exercise," South Sudan's Minister of Information Barnaba Marial Benjamin has said.
Rabbie Abdelattie, a senior official in Sudan's ruling National Congress Party, denied the charges.
He told the BBC that the Sudanese government was providing lots of humanitarian assistance to South Sudanese.
But numerous South Sudanese living in Khartoum told me they believed the authorities in the city are at the very least condoning forced conscription.
One man said his uncle had been snatched: "He is not a soldier, and he has no military experience, so I am worried he won't know how to fight or protect himself."
He assumes his uncle has been sent to a training camp, before he is sent into battle, perhaps never to be seen again.
Like almost everyone else, he was too scared to allow his name to be used.
Others spoke of a priest, his assistant and a driver who had gone missing. Some men are now too scared to seek the casual work they need to pay their bills.
The UN's refugee agency, UNHCR, has good contacts with the South Sudanese communities in Sudan and believes there are 700,000 South Sudanese still living in the north, nearly six months after the southern region seceded.
It has no independent confirmation of the abductions, but is worried by what its sources are reporting.
"They describe that there have been house-to-house searches in some areas," says Philippa Candler, UNHCR Sudan's head of protection.
"We have heard that there is a possibility of paying a ransom, so that those who pay a ransom are released."
UNHCR says it has raised the matter with the Sudanese government.
All the South Sudanese sources - official and unofficial - accuse rebel groups of carrying out the kidnappings.
The name that comes up most often is the South Sudan Liberation Army (SSLA), which mainly operates in oil-rich Unity state.
General Bapiny Monytuil, the SSLA's deputy commander, called the accusations "lies".
He says bus loads of South Sudanese men going to the border are his fighters returning from Khartoum where they had been for rest or to have injuries treated.
"We have wounded men who come for treatment in Khartoum, then we send a mission to come to send them back to the border," he told the BBC.
"Why would we forcibly recruit the citizens? Everyone supports me."
Other South Sudanese rebel groups fight in Upper Nile and Jonglei states. Anger in Upper Nile against the rebels runs deep.
"Their objective is survival, and self-interest. They are fighting for nothing, and they are killing innocent civilians," Simon Kun Pouch, the governor of Upper Nile state told the BBC.
"They go and collect people by force, and send them to the front line by force. I don't know what God will do for these people."
In Khartoum, fear has gripped many South Sudanese.
But one old woman was determined to talk - though she, too, did not want her name to be used.
"We heard that some of the people who were taken have already been killed in the war there," she said, in a soft voice.
"We got a message from other people. The names of the dead have already reached here. So we are warning all the South Sudanese people here in Khartoum."
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.