A contaminant found in a batch of Moderna's COVID-19 vaccines delivered to Japan is believed to be a metallic particle.
That's according to a report by Japanese public broadcaster NHK, citing sources at the health ministry.
Japan on Thursday suspended the use of 1.63 million doses shipped to hundreds of vaccination centers nationwide.
The move came more than a week after the domestic distributor, Takeda Pharmaceutical, received reports of contaminants in some vials.
The NHK report cited ministry sources as saying the particle reacted to magnets and was therefore suspected to be a metal.
Moderna had described it as quote "particulate matter" that did not pose a safety or efficacy issue.
An official at the health ministry said the identity of the contaminant has not been confirmed.
The news could provide a fresh setback for Japan's inoculation drive as it struggles to persuade many - particularly younger people - to get vaccinated.
The ministry said the suspension of the Moderna batches was a precaution.
But the move prompted several Japanese companies to cancel worker vaccinations and the European drugs regulator to launch an investigation.
Spanish pharma company Rovi, which bottles Moderna vaccines for markets other than the United States, said the contamination could be due to a manufacturing issue in a production line.
Moderna put the lot in question and two adjacent ones on hold.
A separate official said it would take some time to confirm how many shots from the contaminated batch had already been administered in Japan.
So far, about 54% of Japan's population has received at least one dose, according to a Reuters vaccine tracker.