A second person has died of Ebola in northwestern Democratic Republic of Congo amid a fresh outbreak of the deadly disease, the World Health Organization (WHO) said on Tuesday.

The patient was a 25-year-old woman who was the sister-in-law of the first case, the WHO said on Twitter. She began experiencing symptoms 12 days earlier, it said.

Genetic testing showed an infection confirmed last week in the city of Mbandaka in the northwest of the country was a new "spillover event." a transmission from an infected animal and not linked to any previous outbreaks, according to the National Institute of Biomedical Research.

The first patient began showing symptoms on April 5, but did not seek treatment for more than a week and died in an Ebola treatment center on April 21. 

The lag time has health workers rushing to identify contacts who may have been infected, the WHO said.

At least 145 people came into contact with the confirmed cases and their health is being closely monitored, the organization said. It later said that one of the first cases was a health worker.

Mbandaka is a trading hub of over 1 million on the banks of the Congo River, in close proximity with road, water and air links to the capital Kinshasa.

Ibrahima Soce Fall, WHO emergencies director for Africa, warned there was a risk the disease could spread to neighbouring Central African Republic and Congo Brazzaville.

"This is concerning but taking into account the capacity build up and experience in Congo we believe it can be contained," Fall said at a press conference in Geneva.

Congo has seen 13 previous outbreaks of Ebola, including one in 2018-2020 that killed nearly 2,300 people, the second highest toll recorded in the history of the hemorrhagic fever.

The most recent outbreak in the east of the country ended in December after six deaths. 

(With input from Reuters)