TOKYO — Japan’s conservative prime minister, Shinzo Abe, sent a message of support earlier this year to a ceremony honoring more than 1,000 Japanese who died after convictions for war crimes, a government spokesman said on Wednesday. The move could worsen frictions between Japan and its neighbors, particularly China, over bitter memories of World War II in the region.

Mr. Abe wrote in the message that the convicted war criminals had “sacrificed their souls to become the foundation of the fatherland,” according to a news report that first appeared in The Asahi Shimbun, a major Japanese newspaper.

The spokesman, Chief Cabinet Secretary Yoshihide Suga, confirmed the report, saying that Mr. Abe had sent the message not in his public capacity as prime minister but as a private citizen, signing his name as the head of the governing Liberal Democratic Party. The report said the message was read aloud at the ceremony, which was held on April 29 at the Koyasan temple in western Japan. Mr. Abe did not attend.

The annual ceremony, which is not well known in Japan, honors 1,180 Japanese who were executed or died in prison after being convicted of war crimes by Allied tribunals. The tribunals, which were held across Asia after the war, convicted the Japanese soldiers and officials for crimes such as the massacre and rape of civilians and the killing of Allied prisoners of war.