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.
