The UN migration agency said on Tuesday it was stepping up efforts to repatriate thousands of South Sudanese people stranded in Sudan, amid escalating tensions between the two countries.
A convoy of 12 river barges carrying more than 3,000 Southern Sudanese left Kosti, south of Khartoum, for Juba, the capital of the newly-independent South Sudan on Monday, the International Organization for Migration said.
"IOM is scaling up its programme to assist South Sudanese move from the north, using river barges, trains and planes to reduce the suffering of thousands who have been stranded for many months awaiting transport assistance to the South," a statement said.
"The returnees are among those who have been stranded at Kosti for many months in need of transport assistance to take them to their final destinations in the South."
More than 17,000 people have made the 1,436-kilometre (892-mile) river journey since the January referendum on independence, the IOM said.
The first in a series of rail shuttles began on Friday, when two trains carrying 2,700 people from Khartoum arrived in Aweil, the capital of Northern Bahr El-Ghazal in South Sudan.
UN chief Ban Ki-moon on Monday raised "deep concern over the escalation of rhetoric" between Sudan and South Sudan in recent weeks.
At least 11 people died in an air raid on the Yida refugee camp in South Sudan on Thursday. Sudan denied it launched the attack.
The IOM expects to assist about 32,000 people return to South Sudan by the end of the year.
The Sudanese government had given the Southern Sudanese living in the north a deadline of nine months from the declaration of independence in July to leave or legalise their stay, it said.
North and South Sudan fought a two decade civil war up to 2005 which left two million people dead. Following their division this year, they have been unable to agree a border and the sharing of revenues and debts.