Health Minister Mitoha Ondo'o Ayekaba said a "health alert" had been declared in the Kie-Ntem province and the neighbouring district of Mongomo, with a "lockdown plan implemented" after consulting with the World Health Organization and the United Nations.
The Marburg virus is a highly dangerous pathogen that causes severe fever that often includes bleeding, often targeting several organs and often reducing the body's ability to function on its own.
It is part of the so-called filovirus family that also includes the Ebola virus, which has wreaked havoc in several previous outbreaks in Africa.
The natural habitat of the Marburg virus is the African fruit bat, which carry the virus but do not fall sick from it.
But the animals can pass the virus to primates in close proximity, including humans, and human-to-human transmission then occurs through contact with blood or other bodily fluids.