Japan is monitoring developments in South Korea with "particular and grave" concerns, the top government spokesman said Wednesday, following the detention of President Yoon Suk Yeol over his declaration of martial law in December.

The importance of bilateral relations remains unchanged despite the latest development and Japan will continue to communicate closely with South Korea, Chief Cabinet Secretary Yoshimasa Hayashi said at a regular press conference.

"Japan views South Korea as an important neighbor who as a partner can jointly tackle global challenges," Hayashi said, adding, "The importance of relations between Japan and South Korea will not change."

While Yoon's abrupt yet short-lived imposition of martial law stunned South Korea and the world last month, relations between Tokyo and Seoul have markedly improved since he became president in 2022.

Japanese Prime Minister Shigeru Ishiba, who took office last October, has expressed his hope that momentum can be maintained to improve ties long fraught with difficulties stemming from Japan's 1910-1945 colonial rule of the Korean Peninsula.

Both countries are U.S. allies and their trilateral cooperation has taken on greater importance in the face of North Korea's nuclear and missile development.

A senior Japanese Foreign Ministry official said the detention of Yoon came at a bad time, just before U.S. President-elect Donald Trump returns to the White House next week.

"The political confusion in South Korea will likely continue for a while," the official said.

Another ministry source said, "Whoever takes the top job (in South Korea), the trilateral framework will remain important. We will maintain our communication at the working level."

Japan's Foreign Minister Takeshi Iwaya and his South Korean counterpart Cho Tae Yul earlier this week underscored the importance of bilateral ties despite the uncertainty caused by Yoon's impeachment last month.

This year marks the 60th anniversary of the normalization of diplomatic ties between Japan and South Korea.