The antiviral drug Avifavir, known generically as favipiravir, was first developed in the late 1990s by a Japanese company later bought by Fujifilm as it moved into healthcare.

Russian hospitals can begin giving the antiviral drug, which is registered under the name Avifavir, to patients from June 11, RDIF head Kirill Dmitriev said Russian scientists had modified and enhanced Avifavir andMoscow would be ready to share the details of those modifications within two weeks.

Avifavir appeared on a Russian government list of approved drugs on Saturday.

Dmitriev said clinical trials of the drug had been conducted involving 330 people, and had shown that it successfully treated the virus in most cases within four days.

The trials were due to be concluded in around a week, he said, but the health ministry had given its approval for the drug's use under a special accelerated process and manufacturing had begun in March.

Clinical trials to test efficacy drugs usually take many months, even when expedited, and involve large numbers of patients randomly assigned who receive either the drug being trialled or a control or placebo.

Success in small small-scale, early-stage trials is no guarantee of success in later, more comprehensive trials.

A study published this month, for example, tied the anti-malarial drug hydroxychloroquine, which U.S. President Donald Trump says he has been taking and has urged others to use, to an increased risk of death in hospitalised COVID-19 patients.

Dmitriev said Russia was able to cut testing timescales because the Japanese generic drug which Avifavir is based on was first registered in 2014 and had undergone significant testing before Russian specialists modified it.

"We believe this is a game changer. It will reduce strain on the healthcare system, we'll have fewer people getting into a critical condition," said Dmitriev. "We believe that the drug is key to resuming full economic activity in Russia."

With 414,878 cases, Russia has the third highest number of infections in the world after Brazil and the United States, but has a relatively low official death toll of 4,855 - something that has been the focus of debate.

RDIF, which has a 50% share in the drug's manufacturer ChemRar, funded the trials and other work with its partners, to the tune of around 300 million roubles ($4.3 million), said Dmitriev, who explained that the costs to Russia were much lower because of previous development work conducted in Japan.