About 30,000 people have fled recent fighting in Sudan's South Kordofan state, the UN said on Friday after rebels captured 29 Chinese workers in the same area.
The United Nations humanitarian agency (OCHA) said tension remains high in the state's northwest around El Abbasiya.
"Humanitarian organisations have reported that displacement is continuing following the build-up of SAF (government) troops in the area, with armed clashes between the SAF and SPLM-N (rebels) expected to take place any time," OCHA said in its weekly humanitarian bulletin.
The Chinese were captured one week ago when the Sudan People's Liberation Movement-North destroyed a Sudanese military convoy between Rashad town and El Abbasiya and took over the area, the rebels said.
"On 28 January 2012, humanitarian organisations reported that SPLM-N forces took control of 10 villages in the El Abbasiya area of South Kordofan" OCHA said.
"An estimated 12,000 people fled these villages to areas northwest of El Abbasiya town," while an additional 18,000 -- almost the entire population of El Abbasiya -- escaped from the town. Many sought shelter in village schools, OCHA reported.
Khartoum has severely restricted the work of foreign relief agencies in South Kordofan and nearby Blue Nile state, where fighting began months ago between the government and ethnic minority insurgents who fought alongside the former rebels now ruling South Sudan.
The South gained independence from Khartoum last July after decades of civil war.
Beijing has dispatched a six-member team to Sudan to help secure the release of the captives, who were involved in a road-building project.
The Chinese have not yet made contact with the rebels, Arnu Ngutulu Lodi, spokesman for SPLM-N in South Kordofan state, told AFP.
He said the Chinese -- whom the Sudanese military refers to as hostages -- are well looked-after.
The UN's OCHA said about 140,000 Sudanese refugees have fled to South Sudan or Ethiopia since fighting began in South Kordofan last June, followed by Blue Nile in September.