The antiviral drug remdesivir, considered one of the most promising Covid-19 treatments, turns out to do little to prevent deaths from the disease, according to a WHO-backed study.

Remdesivir, which was part of the experimental cocktail given to US President Donald Trump when he caught the new coronavirus last month, was one of several reviewed in a large study of more than 11,000 people across 30 countries.

The drug "appeared to have little or no effect on hospitalised COVID-19, as indicated by overall mortality, initiation of ventilation and duration of hospital stay," said the study posted online late Thursday.

The data, which has yet to be peer-reviewed or published in a scientific journal, seems to contradict at least two major US studies that have shown that remdesivir can reduce the duration of hospital stays for COVID-19 patients.

Washington authorised the emergency use of the medicine, made by US pharmaceutical company Gilead Sciences and originally intended as a treatment for Ebola -- on May 1, followed by  a range of other countries and the European Union.