Kinshasa, DR Congo - More than 2,000 Ebola cases, including 754 deaths, have been recorded in the DR Congo, where the World Health Organization warns the outbreak may be two to four times larger than official figures suggest.

The outbreak has spread to five provinces in the central African country, according to the latest figures released by Congolese health authorities on Wednesday.

There is currently no approved vaccine or treatment for the Bundibugyo strain behind this outbreak, but the first clinical trial of an antiviral drug began Tuesday, the WHO said.

NGO Doctors Without Borders (MSF) warned Wednesday that the outbreak was spreading at an unprecedented pace and into new areas, calling for an urgent scaling up of the medical response.

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"In less than five weeks, the number of confirmed cases has tripled," while "the number of deaths has increased more than fivefold," MSF said in a French statement.

The aid group added that the outbreak had already surpassed half the number of cases recorded during the 2018-2020 Ebola epidemic in the DRC, which lasted nearly two years.

WHO emergencies chief Chikwe Ihekweazu said on Tuesday that 80% of new cases were not on known contact lists of cases and came from "unknown chains of transmission."

He also voiced alarm that many newly reported cases are people who died without ever reaching a health facility.

The clinical trial, called EBO-PEP, is evaluating the effectiveness of antiviral obeldesivir in people exposed to confirmed Bundibugyo cases.

The experimental drug, developed by US pharma firm Gilead Sciences, has shown promise in pre-clinical models against filoviruses, the family of viruses that cause haemorrhagic fevers.

The outbreak was declared on May 15 after several cases in Ituri, a mineral-rich northeastern province where armed groups frequently carry out deadly attacks.