The announcement came after a devastating week, when 13 cases were confirmed on Wednesday - a one-day record - and officials confirmed even newborn babies are catching the virus.
Ebola caused global alarm in 2014 when the world's worst outbreak began in West Africa, killing more than 11,300 people and infecting an estimated 28,600 as it swept through Liberia, Guinea and Sierra Leone.
Global health experts are urging the Donald Trump administration to allow United States government disease specialists - "some of the world's most experienced" - to return to northeastern Democratic Republic of Congo to help fight the second-largest Ebola outbreak in history.
Dr Peter Salama, WHO's emergencies chief, called it a "sad toll" as DR Congo's health ministry announced the number of cases has reached 426. Ebola cases have also been recorded in neighboring Ituri province.
Because the data collected in the North Kivu epidemic is unlikely to be sufficient for a complete study, the country's health ministry said the clinical trial may extend over a five-year period to cover Ebola outbreaks in other countries.
Attacks by rebel groups and open hostility by some wary locals have posed serious challenges to health workers that Ebola experts say they've never been seen before.
Many venture out on critical virus containment missions only accompanied by United Nations peacekeepers in areas where gunfire echoes daily.
Salama of World Health Organization predicted that the outbreak in the northeastern part of the country will last at least another six months before it can be contained.
Congo is the second worst country in the world for malaria, trailing only Nigeria, and Hoyer said there were no more mosquito-blocking bed nets left in North Kivu, an eastern province that is battling both conflict and disease.
Testing of four experimental Ebola drugs have begun.
And yet the risk of Ebola spreading in so-called "red zones" - areas that are virtually inaccessible because of the threat of rebel groups - is a major concern in containing this outbreak.
"This tragic milestone clearly demonstrates the complexity and severity of the outbreak".
Health workers practice proper donning of protective gear during the training on vaccination against Ebola.
There are also concerns regarding the high number of young children affected with the virus, according to the WHO. Another 87 cases were under investigation. Two top medical journals this week have published commentaries calling on the U.S.to change its mind and send them back where they are sorely needed.
"It is in US national interests to control outbreaks before they escalate into a crisis", one group of global health experts wrote in a commentary in the Journal of the American Medical Association.
Violence is common in the central African country and has made it hard to stop the Ebola outbreak - earlier in the same week eight United Nations peacekeepers and 12 local soldiers died in an ambush.