TOKYO—Japan said it would phase out imports of Russian coal and expel eight Russian diplomats but keep investing in a $21 billion Siberian natural-gas project under construction.

Prime Minister Fumio Kishida released a list of new sanctions Friday, including asset freezes covering Russia’s largest banks and an import ban on Russian lumber. He said Japan would release 15 million barrels of oil from public and private reserves to help keep down prices, including the first release from the government’s own reserves.

The measures were in line with those previously announced by the U.S. and European nations.

“We must respond with severity to Russia’s barbarous conduct,” Mr. Kishida said, citing what he described as war crimes committed by Russian forces. Russia has denied targeting civilians in the Ukraine war.

According to the state news agency Tass, Russia’s ambassador to Japan, Mikhail Galuzin, criticized Tokyo’s expulsion of diplomats and said it was “aimed at destroying Russian-Japanese relations.” Russia said the explusions wouldn’t go unanswered, according to Tass.

Mr. Kishida reiterated that Japan would keep buying Russian oil and gas, which accounts for the bulk of its imports from Russia. In contrast to Germany, which has said it would end Russian coal imports by summer, Mr. Kishida said he wasn’t setting a target date for the planned coal import ban. Russia accounted for 11% of Japan’s coal imports in 2021.

The Group of Seven industrialized nations, which includes Japan, agreed Thursday to ban new investments in key sectors of the Russian economy including energy. Japan’s minister of economy, trade and industry, Koichi Hagiuda, said the agreement didn’t cover additional investments in existing projects.

As a result, Mr. Hagiuda said, Japan intends to continue its role in the Arctic LNG 2 project under construction in Siberia, led by Russian natural-gas producer Novatek. A public-private Japanese consortium has a 10% stake in the project, which is costing $21.3 billion to build, according to its website. The project will liquefy Russian natural gas so it can be shipped to overseas buyers.