Thousands of South Sudanese stranded for months in a camp in Sudan are to be airlifted home, an aid group says.

At least 12,000 refugees were stuck in the river town of Kosti, in White Nile State, when boat traffic south was cut because of cross-border violence.

They will now be taken by bus to Khartoum and then flown to Juba, South Sudan's capital, the International Organization for Migration said.

The IOM says Sudan has dropped a demand that they leave by 20 May.

Meanwhile, a UN deadline for both South Sudan and Sudan to cease all hostilities or face sanctions expired yesterday. The sporadic unrest has sparked fears of all-out war between the countries.

The IOM said the airlift of the South Sudanese refugees had been made possible after the Sudanese government agreed to help.

It said Khartoum had promised to provide emergency travel documents and arrange for the transport of excess baggage which cannot be accommodated on the charter flights on which the South Sudanese will be flown south.

"We hope to start within a week," Jill Helke, head of the IOM office in Khartoum, told the AFP news agency.

The refugees are among an estimated 350,000 South Sudanese stranded in Sudan after a period of grace for them to regularize their status ended on 8 April, according to the AFP news agency.

After South Sudanese troops briefly occupied the disputed Sudanese border town and oil field of Heglig last month, the governor of White Nile state declared the South Sudanese a security risk and demanded that they leave by 5 May.

He later extended this deadline to 20 May, and has now withdrawn it in response to the IOM's departure plan, the IOM says.