The new outbreak, publicly declared on May 8 with 23 deaths so far, had previously been confined to a remote rural area in Equateur province in the northwest of the country.
But the UN health agency confirmed that the case of Ebola was registered in the city of Mbandaka, about 150 kilometers from the Bikoro area where the outbreak originated.
"This is a worrying development," WHO Director-General Tedros Adhanom Ghebreyesus said in a statement
. Last week, a senior WHO official warned that if the virus reached Mbandaka, the DRC could face with another Ebola crisis.
The population of the city has been estimated between 700 000 and 1.2 million.  "If we see a city of that size infected with Ebola, then we are going to have a big urban outbreak," WHO emergency response chief Peter Salama told reporters last week.
The agency said today it was deploying about 30 experts to Mbandaka "to carry out surveillance in the city", including rapid efforts to track all contacts of the new urban case.
44 cases have been reported so far, including three confirmed, 20 probable and 21 suspects, according to the WHO count.
Ebola is not only lethal but also highly contagious, which hinders its containment and recoil.
Lacking an arsenal of drugs to treat or prevent the virus, doctors use classic tactics to isolate patients and track people who have been in contact with them.
This challenge is greatly amplified in urban environments where people move more and have more contact with others than in the countryside.
In addition to the headache is the fact that we have exploded again in one of the most vulnerable and volatile countries in the world.
A country four times larger than France, the Democratic Republic of the Congo has been chronically unstable and episodically tormented by violence since its independence from Belgium in 1960.  Despite the great mineral wealth, the country remains mired in poverty and has a reputation for corruption and bad governance. The basic infrastructure -hospitals, roads, electricity- is a major obstacle in remote areas.
This is the ninth time that the Democratic Republic of the Congo has been affected by the Ebola virus since 1976, when the deady viral disease was identified for the first time in Zaire by a Belgian. directed team.
The virus is widely considered one of the most terrifying in the world, because it can spread easily and kill quickly.
WHO has also confirmed that the current outbreak is the same strain of the virus that broke out in West Africa in 2013 and killed more than 11,300 people, the deadliest Ebola epidemic ever.
WHO was harshly criticized for its handling of the 2013 outbreak.
Tedros, who took over WHO last year, has promised that improving the response to the crisis would be a key priority for the agency.  "We now have better tools than ever to fight Ebola," he said on Tuesday after visiting the DRC over the weekend to evaluate the response.
A terrible infrastructure in Bikoro, especially a poor road network, has created logistical complications for health workers who try to reach the area.
But this time the WHO response was aided by a new experimental vaccine, which arrived in the DRC on Wednesday and has been approved for use by the Kinshasa government.
Tedros said over the weekend that WHO had enough doses of the vaccine to respond, but it was not immediately clear if the stock was enough to handle a major urban outbreak. – AFP