Japan expressed grave concern about Russia's possible use of nuclear weapons against Ukraine, as the top government spokesman on Monday announced an additional ban on exports of chemical weapons-related goods to Russia.

"As the world's only country to have suffered nuclear attacks, we strongly demand that the threat or use of nuclear weapons by Russia should never happen," Chief Cabinet Secretary Hirokazu Matsuno told a regular news conference.

Matsuno was responding to a question about US national security adviser Jake Sullivan's comments on Sunday on CBS news that the United States will act "decisively" in case of a Russian nuclear attack on Ukraine.

Asked if Japan approves of the possible use of nuclear weapons by the United States against Russia, Matsuno refused to comment because it was a hypothetical question.

On Monday, Japan's government banned the export of materials that may be used for chemical weapons to 21 Russian organizations, including science laboratories. The measure was approved by the Cabinet following a decision by the Group of Seven foreign ministers last week.