North Korea has told Japan it could raise the subject of wartime forced labor during Tokyo’s occupation of the Korean Peninsula in any future bilateral talks, diplomatic sources have said.

The wartime labor issue has been the source of heightened tensions between Tokyo and Seoul after South Korea’s top court last year ordered several Japanese companies to compensate South Korean workers for forced labor.

Japan has refused to comply with the ruling based on its stance that the matter of compensation for wartime labor was resolved under a 1965 agreement that normalized relations with South Korea. Pyongyang has repeatedly criticized this stance in state-run media.

Japan colonized the Korean Peninsula from 1910 until the end of World War II in 1945.

According to the sources, North Korean Foreign Minister Ri Yong Ho warned his Japanese counterpart, Taro Kono, via a Mongolian diplomat in mid-December, that he would have “no choice” but to bring up the matter if Tokyo insists on pursuing the issue of Pyongyang’s abductions of Japanese citizens in the 1970s and 1980s.

Japan is unlikely to back down on the abduction issue, which Prime Minister Shinzo Abe has said is a top priority. One official close to Abe said the administration will pursue a resolution to the issue “no matter what the other side says.”

This means that even if Japan does open a dialogue with North Korea, as South Korea and the United States have done, negotiations will likely be fraught with obstacles.

Ri met with Mongolian Foreign Minister Damdin Tsogtbaatar in Ulan Bator on Dec. 8.

According to the sources, Ri asked Tsogtbaatar to relay the message that Japan was being illogical by asking for the return of individuals it identifies as abductees who have already died or never entered North Korea in the first place.

Ri also said that should Tokyo continue to focus on the abductions, he would bring up the issue of the “more than 8.4 million Koreans who were forced to work” under Japanese colonial rule, the sources said.

Japan passed the National Mobilization Law in 1938, during the Second Sino-Japanese War, allowing it to begin requisitioning workers at home the following year and later on the Korean Peninsula.

Abe said last week that his government is “using various channels” to communicate with North Korea, including over a potential summit with leader Kim Jong Un.

Asked by a reporter about meeting Kim, Abe said last June’s summit in Singapore between Kim and U.S. President Donald Trump meant the “situation has greatly changed” and as both sides work toward further talks, “I think next time around I will also have to face Kim Jong Un.”

For his part, Kim, too, appears interested in talks with Abe, telling South Korean President Moon Jae-in in late April that he is ready to hold a dialogue with Japan at “any time.”

However, Abe has said time after time that Japan will hold back any economic incentives until all of its concerns — the nuclear, missile and abduction issues — are resolved.

For North Korea, some experts say Japan represents a potential cash cow and could play a key role in a claimed shift by Kim from a focus on nuclear weapons to his country’s tattered economy.

For Tokyo, with its long historical links to the Korean Peninsula, its nuclear-armed neighbor represents more than a mere security concern — despite its arsenal of shorter-range missiles capable of striking much of Japan. Rather, the normalization of ties with the North is seen as one of the final pieces in a puzzle Japan is still trying to solve more than 70 years after its colonial rule of the Korean Peninsula ended with its defeat in World War II.