The large government-organised convoy of about 1,700 people was unable to continue beyond Heglig "due to an upsurge of fighting in the area," said Ali al-Zatari, the United Nations resident and humanitarian coordinator in Sudan.

Most of the convoy has returned to a location north of Heglig.

"But there are reports that some separated passengers remain in Heglig and are in urgent need of food, water and shelter," he said, adding the United Nations has offered its assistance to the government.

Heglig town is about 15 kilometres (nine miles) from the closest point of the disputed frontier with South Sudan.

The South said its forces had taken the area on Monday when they pushed back Khartoum's troops which had moved over the frontier into Unity state following air strikes.

But Khartoum said Southern soldiers had pushed into Sudan before being repulsed.

Sudan's area commander for Heglig, Bashir Meki, told a visiting Khartoum delegation on Wednesday that the area is completely secure after recent fighting, the signs of which were still visible.

The stranded returnees are among an estimated 500,000 ethnic South Sudanese remaining in the north before an April 8 deadline for them to leave or regularise their status at the end of a grace period following the South's independence in July.