The World Health Organization (WHO) says there's a "high risk of biological hazard" at a laboratory caught up in the ongoing conflict in Sudan.
Officials said it was unclear who was behind the occupation of the National Public Health Laboratory in the capital Khartoum.
The city has been ravaged by fighting between the army and the paramilitary Rapid Support Forces (RSF).
The WHO told the BBC on Tuesday that workers can no longer access the lab.
It warned power cuts were making it impossible to properly manage material at the lab.
Officials said a broad range of biological and chemical materials were stored in the lab.
The facility holds measles and cholera pathogens, as well as other hazardous materials.
A lack of power is also putting depleting stocks of blood bags stored at the lab at risk of spoiling.