TOKYO (Kyodo) -- Japan on Friday expressed serious concern about the reported dispatch of North Korean troops to Russia for potential participation in the war in Ukraine.

Chief Cabinet Secretary Yoshimasa Hayashi also said the recent deepening of military cooperation between North Korea and Russia is worrying, given its impact on the security of the region surrounding Japan.

"The government is monitoring with serious concern North Korea's dispatch of troops to Russia, including the possibility of the troops taking part in Russia's invasion of Ukraine," Hayashi, the top government spokesman, said at a regular press briefing.

North Korean soldiers had moved to the western Russian region of Kursk near the Ukrainian border after completing training, Ukraine said Thursday.

Russian President Vladimir Putin did not deny that North Korean troops had been sent to Russia, as claimed by the U.S. and South Korean governments based on satellite imagery and other intelligence.

"If there are images, then they reflect something," Putin told a press conference in the Russian city of Kazan on Thursday.

Japan, a member of the Group of Seven countries, has imposed a series of sanctions on Russia since it launched its full-scale invasion of Ukraine in 2022.

"It's deeply concerning that recent progress in the deepening military cooperation between Russia and North Korea will worsen the situation in Ukraine and impact the security of the region around Japan," Hayashi said.

Japan will work with other nations to bring peace to Ukraine in a "fair and lasting manner" at an early date, he added.