The World Health Organization said it has confirmed sexual transmission of mpox in Congo for the first time as the country experiences its biggest-ever outbreak, a worrying development that African scientists warn could make it more difficult to stop the disease.

In a statement issued late Thursday, the U.N. health agency said a resident of Belgium traveled to Congo in March and tested positive for mpox, or monkeypox, shortly afterward. WHO said the individual “identified himself as a man who has sexual relations with other men” and that he had gone to several underground clubs for gay and bisexual men.

Among his sexual contacts, five later tested positive for mpox, WHO said.