Japan and South Korea are growing closer amid common threats, such as Pyongyang's nuclear program. Japanese Prime Minister Fumio Kishida's visit to Seoul comes after South Korea's leader made a trip to Tokyo in March.

Deepening South Korean and Japanese ties should not be hindered by unresolved historical disputes, South Korean President Yoon Suk Yeol said on Sunday during a summit in Seoul with Japanese Prime Minister Fumio Kishida in Seoul.

Kishida, meanwhile, said "his heart aches" over Korea's colonial-era suffering during Japanese occupation.

The summit comes after Yoon made a similar visit to Tokyo in March, as both countries aim to strengthen ties amid growing regional threats from North Korea and China.

Both leaders vowed to boost cooperation and coordination. Yoon said this would not only serve their countries' common interests but would also be essential "for world peace and prosperity in the face of the current severe international situation."

Kishida meanwhile invited Yoon to the Group of Seven Summit, scheduled in Japan later this month. He also asked him to join trilateral talks with the US due to be held on the sidelines.

On the countries' sensitive histories, the Japanese prime minister expressed his sympathy for the Koreans who suffered during Japan's occupation, yet with words short of clearly addressing the Japanese occupation.

"My heart aches as many people went through a very difficult and sad experience in the harsh environment at that time," he said.

At the start of the trip, Kishida was greeted by officials on his arrival and visited the Seoul National Cemetery to lay flowers for South Korea's war veterans. The cemetery includes graves of those who fought in the Korean War, as well as those of Korean independence fighters who died during Japanese rule over the peninsula.

Kishida then went into talks with Yoon at the presidential office in Seoul.

The Japanese leader further said relations between Tokyo and Seoul have been "moving forward dynamically" since Yoon's visit in March.

The South Korean President, in turn, said he felt it was his "responsibility to make ties with Japan better."

The two Asian nations — key allies of the United States in their tensions with North Korea — have long had lukewarm relations because of Japan's colonial occupation of the peninsula for 35 years until 1945. Japan's colonial-era atrocities towards Korea, such as sexual slavery and forced labor, have made reconciliation difficult.

President Yoon is now attempting to repair ties and has made it a priority for his administration. Yoon visited Japan in March, attempting to open a new chapter in relations between the two countries despite historical feuds.

Ahead of Kishida's visit to South Korea, the Japanese prime minister told reporters that he wishes to have an "open-hearted exchange of views" with Yoon.

Few experts, however, expect Kishida to apologize further for the atrocities towards South Koreans, even though they believe Japan has not apologized sufficiently.

In 2015, the two countries reached a settlement under which Tokyo issued an official apology to "comfort women" who say they were enslaved in wartime brothels, and provided 1 billion yen ($7.4 million; €6.6 billion) to a fund to help the victims. The fund was soon scrapped by South Korea for being an insufficient help to victims.

Meanwhile around 100 South Koreans staged a protest against Kishida's trip on Saturday, saying that colonial wrongdoings must be at the top of the summit's agenda.

Yoon has signaled that an apology from Kishida will not be necessary. Instead, he is likely to focus on their shared cooperation in the face of North Korea's nuclear threat, Shin-wha Lee, an international relations professor at Korea University told Reuters.

US President Joe Biden recently hosted Yoon at the White House, where they signed the "Washington Declaration." Under this, the US pledged to share more information of its nuclear planning and response to Pyongyang with South Korea. In return, Yoon pledged his country would not pursue its own nuclear program.

North Korea's Supreme Leader Kim Jong Un last year declared his country an "irreversible" nuclear power and has since doubled down on weapons development and testing.