The outbreak has reached the city of Mbandaka, about 150km from where it was first reported, according to WHO.

The Ebola outbreak in the Democratic Republic of the Congo has spread to a city, fuelling concern the deadly virus may prove tougher to contain, the World Health Organization (WHO) said on Thursday.

The new outbreak, publicly declared on May 8 with 23 deaths so far, had previously been confined to a very remote, rural area in Equateur Province in the northwest of the country.

But the UN’s health agency confirmed on Thursday that an Ebola case had been recorded in the city of Mbandaka, which lies roughly 150km from the Bikoro area where the outbreak originated.

“This is a concerning development,” WHO Director-General Tedros Adhanom Ghebreyesus said in a statement.

Last week, a top WHO official warned that if the virus reached Mbandaka, DR Congo could be confronted with yet another Ebola crisis.

The city’s population has been variously estimated at between 700,000 and 1.2 million.

“If we see a town of that size infected with Ebola, then we are going to have a major urban outbreak,” the WHO’s head of emergency response, Peter Salama, told reporters last week.

The agency said on Thursday it was deploying around 30 experts to Mbandaka “to conduct surveillance in the city,” including rapid efforts to trace all the people the confirmed urban case had been in contact with.

Forty-four cases have been reported in the outbreak so far, including three confirmed, 20 probable and 21 suspected, according to the WHO’s tally.