Photo taken May 24, 2021, shows a syringe being filled with COVID-19 vaccine developed by Moderna Inc. in Sendai, northeastern Japan. (Kyodo) ==Kyodo

Japanese drugmaker Takeda Pharmaceutical Co., which is in charge of sales and distribution of the vaccine in the country, said it has yet to see any reports of safety concerns.

The size of foreign substances confirmed in 39 vials is believed to be a few millimeters, with their elements being unknown. Moderna is conducting an investigation into the materials, according to the ministry and Takeda.

The Ministry of Health, Labor and Welfare said foreign substances have been confirmed since Aug. 16 at eight vaccination sites in the prefectures of Ibaraki, Saitama, Tokyo, Gifu and Aichi. Takeda reported them to the ministry on Wednesday.

Takeda has requested an emergency probe by the U.S. biotechnology firm, while urging medical institutions and other entities not to use any doses of the vaccine showing abnormality, even for doses not subject to the suspension.

The Japanese company added it will make efforts to prevent the suspension from affecting the availability of coronavirus vaccine doses in the country.

The health ministry said the 1.63 million doses of Moderna vaccine were produced in the same production line at the same time in Spain, and fall under three lot numbers -- 3004667, 3004734 and 3004956.

As these doses have already been distributed to 863 vaccination centers across the country, the ministry will ask each facility not to use them.

Meanwhile, the ministry revealed the lot numbers so that concerned individuals can check whether they may have received a potentially contaminated shot before the affected vaccine doses were suspended.

The problem came to light at a time when Japan is struggling to contain a spike in coronavirus infections, with the government announcing a plan Wednesday to place eight more prefectures under its COVID-19 state of emergency.

While vaccinations against COVID-19 are under way in Japan, the highly contagious Delta variant of the coronavirus has sent daily reported cases surging in many parts of the country.

Over 10 million doses have already been used in the country since the Moderna vaccine was approved for emergency use in May, according to the government.

The Japanese government signed a contract with Moderna to receive 50 million doses of the COVID-19 vaccine by the end of September. Currently people aged 12 and over can receive shots of the vaccine.

The vaccine, like the one developed by U.S. pharmaceutical firm Pfizer Inc. and its German partner BioNTech SE, uses a new technology called messenger RNA, or mRNA, and is administered in two doses given four weeks apart.