A Catholic pilgrim prays at the Church of Nativity, traditionally believed by Christians to be the birthplace of Jesus Christ, in the West Bank town of Bethlehem on Monday. Thousands of Christian worshippers and tourists arrived in Bethlehem to mark Christmas. Adel Hana / Associated Press

From homes of mud brick or roughly built shelters, Sudan's displaced gathered on the sandy lot of St Bakhita's parish church on Monday for Christmas Mass.

The metal benches beneath the church's sagging ceiling were unable to hold all the worshippers. Some were South Sudanese still waiting to go home, and many others ethnic Nuba from war-torn South Kordofan state.

To make room, prayers were held outside near a giant metal cross.

"It will be a very good celebration," a community worker said earlier, despite little reason to rejoice in the Jaborona settlement, which grew out of the desert near Khartoum's twin city of Omdurman during Sudan's 1983-2005 civil war.

One church leader said of the South Sudanese remaining in Islamist-run Sudan, without regular jobs or homes of their own as their cash disappears, "We can survive and we can smile ... but there are a lot of tears in our hearts."

The community worker said, "Most of the southerners who lived in Jaborona left earlier, but about 1,000 are still encamped there in tent-like shelters awaiting transport south."

Similar "departure points" all over the Khartoum area hold what local leaders estimate to be 40,000 South Sudanese.

The civil war drove millions to the north. After South Sudan separated in July 2011, southerners there were given a deadline of April to formalize their status in the north or leave.

South Sudan's embassy says that, at the last count, there were 171,000 South Sudanese still in the Khartoum area.

Sudan and South Sudan have not come up with a detailed plan for returning the South Sudanese, and disagreements have stalled implementation of key deals signed in September on security and economic issues.

These included a pact on the right of each country's nationals to live and move freely in the other country.

There have been small-scale organized returns this year, including one last week by the two governments and the Africa Inland Church which moved more than 900 people by road to South Sudan, said Filiz Demir, of the International Organization for Migration.

On Monday, Christmas Eve, the IOM was resuming flights of sick, elderly and other "extremely vulnerable" South Sudanese, Demir said. The airlift from Khartoum to Aweil in South Sudan will continue until Thursday.

In Arabic, "Jaborona" means taken by force. The settlement developed when people displaced by the civil war in the Nuba mountains and south Sudan were moved there by the government.

At its peak it held about 30,000 southerners, but now only about 1,000 remain, while others stay with relatives and return to Jaborona when they hear of transport, the community worker said.

"I think the main problem at the moment is the living conditions of the people," he said, asking for anonymity. "Many young people are just drinking, living a reckless life, and don't go to school."

South Sudanese have been classed as foreigners in Sudan since April, restricting their access to employment and services.

They lost their jobs and sold their homes in expectation of leaving, the community worker said, adding that some women brew and sell alcohol to scrape a living.

Kau Nak, deputy head of South Sudan's embassy in Khartoum, said, "I'm sure their priority is not how to celebrate Christmas, but how to survive and how to transport themselves home."

An estimated 100,000 Nuba live slightly better-off in Jaborona's rough mud-brick houses spread across a vast expanse of sand.

Agence France-Presse