Two people, both in their 30's, died after receiving Moderna m-RNA COVID-19 vaccine shots from a batch whose use was later suspended after it was found that they may have been contaminated, Japan's health ministry said on Saturday.

Both men died this month within days of receiving their second Moderna doses, the ministry said in a release. Each had a shot from one of three manufacturing lots suspended on Thursday.

A causal link between the vaccine and the deaths has not yet been established, according to the ministry and Takeda Pharmaceutical Company, which distributes the Moderna vaccine in Japan.

"At this time, we do not have any evidence that these deaths are caused by the Moderna Covid-19 vaccine, and it is important to conduct a formal investigation to determine whether there is any connection," Moderna and its Japanese distributor Takeda said in a joint statement on Saturday.

The nature of the particles found in the vials, which were manufactured by Spanish pharmaceutical firm ROVI in Spain, is also not known yet.

The contaminants found in some vials in Japan are likely to be metallic particles, public broadcaster NHK reported, citing health ministry sources.

Japan suspended the use of about 1.63 million doses of the Moderna vaccine shipped to 863 vaccination centres nationwide on August 26 as a precautionary measure after foreign substances were found in some vials.

Around 44 percent of Japan's population has been fully vaccinated.More than 15,800 people have died from Covid-19 in Japan as the country battles yet another outbreak of Coronavirus