Health officials said the newest case was worrying. Fourteen cases have been confirmed with laboratory tests.
Chief executive Doctor Tedros Adhanom Ghebreyesus visited the affected Bikoro area last weekend, saying he hoped for a "better way out" of the latest outbreak in DRC.
"The confirmed case in Mbandaka, a large urban centre located on major national and worldwide river, road and domestic air routes increases the risk of spread within the Democratic Republic of the Congo and to neighbouring countries", the World Health Organization said.
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"We expect an increase in the number of cases confirmed in the coming days and weeks, so we are stepping up our intervention efforts because medical teams on the ground will need more support and capacity and increased access to medicines and equipment", health ministry spokeswoman Jessica Ilunga told Al Jazeera.
WHO-partner, MSF, has also deployed its teams and is working with United Nations agencies to strengthen health capacity to treat Ebola patients.
"The arrival of Ebola in an urban area is very concerning and World Health Organization and partners are working together to rapidly scale up the search for all contacts of the confirmed case in the Mbandaka area", said Dr. Matshidiso Moeti, the WHO's regional director for Africa.
Two suspected cases of hemorrhagic fever were reported in the Wangata health zones that include Mbandaka, the capital of northwestern Equateur province. It is paramount to trace the suspect case in order to have a clearer view on how it reached the city.More news: U.S. senate passes the bill in bid to retain net neutrality
The aid group Medecins Sans Frontieres said 514 people who are thought to have been in contact with infected people were now being monitored.
Isolation zones have been set up in Mbandaka's main hospital as well as Bikoro hospital, with more Ebola treatment centers being set up in Mbandaka and Bikoro. It will be used to protect health workers and "rings" of contacts around each case.
The largest of these outbreaks took place in West Africa from 2014 through 2016 when Ebola killed more people than in all previous outbreaks combined: 11,310.
This is a major development in the outbreak.More news: As criticism for Israel mounts, Guatemala opens its embassy in Jerusalem
The WHO said the risk of the disease spreading was "high" and announced it was preparing for the "worst case scenario". Since then, an experimental vaccine has been developed, and 4,000 doses have arrived in the Congo, with thousands more to follow.
"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 virus was initially transmitted to people from wild animals, including bats and monkeys.
"The arrival of Ebola in an urban area is very concerning and WHO and partners are working together to rapidly scale up the search for all contacts of the confirmed case in the Mbandaka area", WHO Regional Director for Africa, Matshidiso Moeti, said in a statement.More news: Meryl Streep to Star in Panama Papers Thriller for Steven Soderbergh
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