The agency said it was deploying around 30 experts to Mbandaka "to conduct surveillance in the city".
Previous reports of the disease had all been in remote areas where Ebola might spread be more easily contained.
Mr Salama, the WHO's deputy director-general for emergency response, said Mbandaka's location on the Congo river raised the prospect of Ebola spreading to Congo-Brazzaville and the Central African Republic, as well as downstream to Kinshasa, which has a population of 10 million.
Emergency meeting called as Ebola spreads to Congolese city - United Nations health agency The World Health Organization (WHO) is convening an emergency meeting on Friday to "consider the worldwide risks" of the latest outbreak of the deadly disease Ebola, which has now moved to an urban area of the Democratic Republic of the Congo (DRC). Until Thursday, the more than 40 suspected or confirmed cases were all located in the area around Bikoro, close to the Congo River, and around 150 kilometres (about 95 miles) from the provincial capital Mbandaka, which is a busy port city.
Tedros Adhanom Ghebreyesus called it a concerning development.
The number of confirmed Ebola cases of the Democratic Republic of Congo (DRC) has risen to 14 after the Health Ministry announced 11 new confirmed cases and one new related death. Ebola is fatal in about 50 per cent of cases.
"This puts a whole different lens on this outbreak and gives us increased urgency to move very quickly into Mbandaka to stop this new first sign of transmission", he said.
The outbreak in West Africa that started in 2014 was the worst ever recorded.
Ebola survivors often suffer from long-term consequences of the disease and may carry dormant viruses. Its early symptoms are not obvious and the worst effects may take weeks to show. On Wednesday, the World Health Organization delivered 4,000 injections of an experimental vaccine with proven efficacy in recent trials, with more batches expected soon.
It said the new death has epidemiologic ties to another case.
Persistent rain and lack of roads has hampered the effort to contain the outbreak so far.
Part of the difficulty in deploying the vaccine is that it must be transported and stored at between minus-60 and minus-80 degrees Fahrenheit, which requires powerful refrigerators. "And we are taking it very seriously", Dr. Ibrahima-Soce Fall, the World Health Organization regional emergency director for Africa, said Monday. Another lethal case in Wangata on Thursday was a suspected one, Xinhua news agency reported.
Congolese authorities declared the outbreak in the northwest region near Congo-Brazzaville on May 8, and three have died from the disease, according to an official toll.
"As more evidence comes in of the separation of cases in space and time, and healthcare workers getting infected, and people attending funerals and then travelling quite big distances - it's got everything we would worry about", he told Reuters.