GENEVA — The World Health Organization said Tuesday it is “cautiously optimistic” about efforts to curb the spread of Ebola in an urban area in Congo, although the lethal virus is still reported in at least two remote areas.

There have been 35 confirmed cases, including 12 deaths. The U.N. health agency and partners have vaccinated more than 400 people with an experimental Ebola vaccine, the first time it has been used in an emerging outbreak.

WHO emergencies chief Peter Salama told reporters that the response has gone “quite smoothly” and that the agency’s first priority had been to stop Ebola’s spread in Mbandaka, a city of more than 1 million that sits on a river described as a “highway” connecting it to other regions. “We can’t conclude we have safeguarded the city of Mbandaka, but so far there hasn’t been an explosive increase in cases…This is a work in progress,” Salama said.