There is a “high risk of biological hazard” in Sudan’s capital Khartoum after one of the warring parties seized a laboratory holding measles and cholera pathogens and other hazardous materials, the World Health Organization said on Tuesday.

Speaking to reporters in Geneva via video link from Sudan, the WHO’s representative in the country, Nima Saeed Abid, said technicians were unable to gain access to the National Public Health Laboratory to secure the materials.

“This is the main concern: no accessibility to the lab technicians to go to the lab and safely contain the biological material and substances available,” he said, declining to specify which side had seized the facility.

Fighting erupted between the Sudanese armed forces and Rapid Support Forces (RSF) paramilitaries on April 15 and has killed at least 459 people and injured 4,072, according to the WHO’s latest figures. Abid said this was an undercount, adding that he had seen two bodies in the street himself in recent days.

The clashes have paralysed hospitals and other essential services, and left many stranded in their homes with dwindling supplies of food and water. The WHO has reported 14 attacks on health facilities since the clashes began and is relocating its staff to safety.

Abid said he was transferred from Khartoum to Port Sudan on Monday as part of a large convoy that drove for 30 hours through the desert.

The United Nations humanitarian office (OCHA) has been forced to cut back on some of its activities in parts of Sudan due to intense fighting.

At least five aid workers have been killed since fighting broke out and the two U.N. agencies who lost staff, the International Organization for Migration and the World Food Programme, have suspended their activities.

“In areas where intense fighting has hampered our humanitarian operations, we have been forced to reduce our footprint,” said Jens Laerke, spokesperson for the OCHA. “But we are committed to continue to deliver for the people of Sudan.”

Patrick Youssef, the International Committee of the Red Cross (ICRC) Regional Director for Africa, urged other countries to continue pressure on Sudan to find a “long-lasting solution”, even after foreigners had been evacuated.