Rebels in Sudan are holding captive six Sudanese civilian engineers who worked with a group of Chinese freed by the rebels several days ago, a spokesman for the insurgents said on Saturday.

The engineers were initially suspected to be security agents working in the war-torn state of South Kordofan, Arnu Ngutulu Lodi, of the Sudan People's Liberation Movement-North (SPLM-N), told AFP.

"They are engineers," he said. "According to the information I got... they are not security."

Lodi said the engineers are fine, and the rebels are prepared to release them.

On Tuesday, the 29 Chinese workers held by SPLM-N for 11 days were released and flown to Kenya on a Red Cross aircraft from South Kordofan, where rebels since June have been fighting with government troops.

Both the Sudanese and the Chinese were captured on January 28 when the SPLM-N destroyed a Sudanese military convoy between Rashad town and Al-Abbasiya and took over the area, the rebels said.

Sudan's military said the workers had been taken hostage.

Beijing lodged a formal protest with Khartoum over their capture and dispatched a six-member team to help gain their freedom.

China's official news agency said the workers, who were involved in a road-building project, were taken after a rebel attack on their camp.

Prior to the release of the Chinese, fighting between rebels and government troops continued between Rashad town and Al-Abbasiya, the United Nations humanitarian agency said on Thursday.

"Injured people were reportedly transported to Um Ruwaba (North Kordofan) for treatment," it said.

The UN has backed statements by the United States that there could be a famine unless urgent aid is allowed to enter South Kordofan and nearby Blue Nile state, where Sudan has severely restricted the work of foreign relief agencies.