KINSHASA, Congo — Congo’s Ebola outbreak has spread to a crossroads city of more than 1 million people in a troubling turn that marks the first time the vast, impoverished country has encountered the lethal virus in an urban area.

“This is a major, major game-changer in the outbreak,” Dr. Peter Salama, the World Health Organization’s deputy director-general of emergency preparedness and response, warned on Thursday.

A single case of Ebola was confirmed in Mbandaka, a densely populated provincial capital on the Congo River, Congo’s Health Minister Oly Ilunga said late Wednesday. The city is about 150 kilometers (93 miles) from Bikoro, the rural area where the outbreak was announced last week.

Late Thursday, Congo’s Ministry of Health announced 11 new confirmed Ebola cases and two deaths tied to cases in the country’s northwest, including one in a suburb of Mbandaka.