KINSHASA, Congo — Three new cases of the often lethal Ebola virus have been confirmed in a city of more than one million people, Congo’s health minister announced, as the spread of the hemorrhagic fever in an urban area raised alarm.

The statement late on Friday said the confirmed cases are in Mbandaka, a city where a single case was confirmed earlier in the week.

There are now 17 confirmed Ebola cases in this outbreak, including one death, plus 21 probable cases and five suspected ones.

The World Health Organization on Friday decided not to declare the outbreak a global health emergency, but it called the risk of spread within Congo “very high” and warned nine neighboring countries that the risk to them was high. The W.H.O. said there should be no international travel or trade restrictions.