SEOUL, May 7 (Reuters) - Japan's Prime Minister Fumio Kishida told South Koreans on Sunday his "heart hurts" when he thinks of suffering and pain during Japanese colonial rule, in a nod to historical disputes that have soured relations between the two U.S. allies.

Kishida was in Seoul for the first visit to the South Korean capital by a Japanese leader in 12 years, returning the trip South Korean President Yoon Suk Yeol made to Tokyo in March where they sought to close a chapter on the historical disputes that have dominated Japan-South Korea relations for decades.

Speaking to reporters after his summit with Yoon, Kishida stopped short of offering a new official apology for wrongs committed under the 1910-1945 occupation, but said his government inherits the stance of earlier administrations, some of which have issued apologies.

"For me personally, my heart hurts when I think of the many people who endured terrible suffering and grief under the difficult circumstances of the time," he said.

Yoon said unresolved historical issues should not mean that no forward steps can be taken to deepen ties.

A senior official at Yoon's office said Kishida's remarks had not been pre-arranged and Yoon thanked him for "showing his sincere position even though there was no such request," and said it would be "greatly helpful for future cooperation."

The pledge by the leaders to boost cooperation has been welcomed by the United States as a way to better confront threats from North Korea and competition from China.

"Cooperation and coordination between South Korea and Japan are essential not only for the common interests of the two countries, but also for world peace and prosperity," Yoon said in opening remarks at their meeting.

Yet South Korea's main opposition Democratic Party criticised Yoon for being submissive, "oblivious to history", and engaging in "humiliation diplomacy."

"Why should the prerequisite for restoring the bilateral diplomacy be giving up on our history," party spokesperson Kang Sun-woo told a briefing, according to Yonhap news agency.