A clinical trial of four potential Ebola treatments in Democratic Republic of Congo has been narrowed to focus on two experimental drugs after they showed "clearly better" results, scientists leading the trial said on Monday.

The study will now focus on Regeneron's REGN-EB3 and a monoclonal antibody called mAb114.

Two other experimental treatments — ZMapp, an intravenous treatment made by Mapp Biopharmaceutical, and Remdesivir, made by Gilead Sciences — will be dropped, the researchers said.

"The two products — mAb114 and Regeneron's — clearly are better than the other two," said Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases (NIAID), part of the U.S. National Institutes of Health and a co-sponsor of the trial.

Ebola virus disease has been spreading in eastern Congo since August 2018 in an outbreak that has now become the world's second largest in history, killing at least 1,800 people.

Efforts to control it have been hampered by militia violence and some local resistance to outside help.

The treatment trial, which began in November last year, is being carried out by an international research group coordinated by the World Health Organization.

Some 681 patients at four separate treatment centers in Congo have already been enrolled, Fauci said. The study aims to enroll a total of 725.

The decision to drop two of the potential treatments was based on data from almost 500 patients, he said, which showed that those who got REGN-EB3 or mAb114 "had a greater chance of survival compared to those participants in the other two arms".

Fauci described Ebola as a "terrible disease" and said that as of now, all future patients in the treatment trial should be randomized to receive either the Regeneron product or mAb114.