A man walks by a house hit in recent fighting in Khartoum on Tuesday. AP

UN officials said on Tuesday that one side in the Sudan conflict has seized control of a national health lab in the capital of Khartoum that holds biological material, calling it an "extremely dangerous” development.

The announcement came as officials warned that more refugees could flee Sudan despite a cease-fire between rival forces.

Dr Nima Saeed Abid, the World Health Organisation's representative in Sudan, expressed concerns that "one of the fighting parties” - he did not identify which one - had seized control of the central public health laboratory in Khartoum and "kicked out all of the technicians."

"That is extremely, extremely dangerous because we have polio isolates in the lab. We have measles isolates in the lab. We have cholera isolates in the lab,” he told a UN briefing in Geneva by video call from Port Sudan. "There is a huge biological risk associated with the occupation of the central public health lab in Khartoum by one of the fighting parties.”

The expulsion of technicians and power cuts in Khartoum mean "it is not possible to properly manage the biological materials that are stored in the lab for medical purposes," WHO said.

The lab is located in central Khartoum, close to flashpoints of the fighting that pits Sudan's military against the Rapid Support Forces, a paramilitary group that grew out of the notorious Janjaweed militias implicated in atrocities in the Darfur conflict.

A US-brokered ceasefire between Sudan's warring generals was largely holding in much of the capital on Tuesday, but witnesses in some areas reported ongoing clashes between the army and paramilitary forces.

Foreign nations stepped up efforts to evacuate their nationals from the chaos-torn nation.

Ten days of heavy fighting until Monday - including air strikes and artillery barrages - have killed hundreds of people, many of them civilians, and left some neighbourhoods of greater Khartoum in ruins.

Bewildered civilians were seen walking down one street in Khartoum North where almost all buildings were bombed out and smoke was rising from scorched ruins, in unverified video posted on social media.

Greek nationals from Sudan arrive with a military C-27 plane at the military airport of Elefsina, south of Athens, on Tuesday.

Since the outbreak of fighting on April 15, at least 20,000 Sudanese have fled into Chad.

Some 4,000 South Sudanese refugees who had been living in Sudan have returned to their home country, UN refugee agency spokeswoman Olga Sarrado said.

The figures could rise, she cautioned.

Sarrado did not have numbers for the five other countries neighbouring Sudan, but the UNHCR has cited unspecified numbers of those fleeing Sudan arriving in Egypt.

"The fighting looks set to trigger further displacement both within and outside the country,” she said, speaking at a UN briefing in Geneva.

The UNHCR was scaling up its operations, she said, even as foreign governments have raced to evacuate their embassy staff and citizens from Sudan.

Many Sudanese have desperately sought ways to escape the chaos, fearing late their all-out battle for power once evacuations are completed.

Several previous cease-fires have failed, although intermittent lulls during the Eid Al Fitr holidays allowed for dramatic evacuations of hundreds of diplomats, aid workers and other foreigners by air and land.

More than 800,000 South Sudanese refugees live in Sudan, a quarter of them in the capital of Khartoum, where they are directly affected by the fighting.

Overall, Sudan hosts 1.1 million refugees, according to the UNHCR. There are also more than 3 million internally displaced persons, mostly in Darfur, a region mired in decades-long conflict, it said.

Along with the refugees, the UN migration agency said there are 300,000 registered migrants, as well as tens of thousands of unregistered migrants in the country.

The UN Population Fund has said that the fighting threatens tens of thousands of pregnant women, including 24,000 women expected to give birth in the coming weeks.

For 219,000 pregnant women across the country it is too dangerous to venture outside their homes to seek urgent care in hospitals and clinics amid the clashes, the agency said.

Dozens of hospitals have shuttered in Khartoum and elsewhere across the country due to the fighting and dwindling medical and fuel supplies, according to the Sudanese Doctors’ Syndicate.

"If the violence does not stop, there is a danger that the health system will collapse,” the UN agency warned Friday.

The International Committee of the Red Cross welcomed the announced cease-fire as a "potential lifesaver for civilians” trapped in their homes in fighting-hit areas.

"It’s clear that this ceasefire must be implemented up and down the chain of command and that it must hold for it to give a real respite to civilians suffering from the fighting,” said Patrick Youssef, ICRC’s regional director for Africa.

He called on the international community to help find a "durable political solution to end the bloodshed.”

Spokesman Jens Laerke of the UN Office for the Coordination of Humanitarian Affairs, said it has been forced to "reduce our footprint” because of the fighting. He pointed to "acute shortages of food, water, medicines and fuel and limited communications and electricity” and new reports of looting of humanitarian warehouses and aid stockpiles.

"The humanitarian needs in Sudan were already at record levels before this recent eruption of fighting … some 15.8 million people - that’s about a third of the population - required humanitarian assistance,” he said.

Agencies