GENEVA — An outbreak of the deadly Ebola virus in rural parts of the Democratic Republic of Congo has spread for the first time to a major city there, the World Health Organization reported on Thursday, raising the threat of a far larger contagion.

The organization said it would convene its emergency committee on Friday to assess the new threat from the disease, which devastated West Africa a few years ago.

Twenty-three people have died in the current outbreak, which started in remote rural areas of Equateur Province, in the northwestern region of the Democratic Republic of Congo, in early April.

The World Health Organization said the health authorities in the country had confirmed that one person was infected with the disease in Mbandaka, the provincial capital, which has a population of more than one million.