A refugee camp was the target of an air strike in South Sudan's Unity state on Thursday, less than 50 km (30 miles) from the violence-ridden border with Sudan, officials and witnesses said.

There were no immediate reports of any casualties.

South Sudan split off into a separate country in July after voting overwhelmingly for secession in a January referendum, the culmination of a 2005 peace deal that ended decades of war between north and south.

Violence along the poorly defined border has raised tensions between the old civil war foes since then. They have accused the other of backing rebel groups on their side of the border.

A Reuters correspondent heard a large explosion in the Yida refugee camp on Thursday, then saw a crater about two metres (6.6 feet) wide, an unexploded bomb wedged in the side of a school building and a white aircraft flying north. Witnesses said there were three other explosions at 3 p.m. (1200 GMT).

Sudan's military spokesman was not immediately available to comment. Taban Deng, governor of Unity state, accused Khartoum of carrying out the attack.

These people (Khartoum) should be taken to book. They should adhere to international laws and regulations, he told reporters in Bentiu.

The refugees need to be safe and need to be protected. They ran away from war. They should not be pursued inside the territory of South Sudan.

Yida is a camp of about 20,000 refugees from the Nuba mountains region of South Kordofan, a state on the north side of the border that has been stricken by conflict between rebels and Sudan's army since June.

(Reporting by Hereward Holland; Additional reporting by Louis Charbonneau and Khalid Abdelaziz; Writing by Alexander Dziadosz; Editing by Mark Heinrich)