A trilateral summit between Japan, South Korea and China most likely will not be held this year as Tokyo has given notice that Prime Minister Yoshihide Suga will not attend without concessions from Seoul in a feud over compensation for wartime labor, diplomatic sources said Monday.

South Korea was slated to host the trilateral summit, which has been held roughly once a year since 2008 as a forum to discuss economic cooperation as well as regional issues including North Korea.

But according to the sources, Japan had told South Korea by late September it would be "impossible" for Suga to visit unless "proper measures" were taken to address its concerns.

Relations between Tokyo and Seoul have slumped to the worst level in years following a South Korean top court decision in October 2018 ordering a Japanese company to pay four men for forced labor during the 1910-1945 period of colonial rule on the Korean Peninsula.

Japan argues the ruling goes against a 1965 bilateral agreement under which it provided South Korea with financial aid with the understanding the compensation issue was settled "completely and finally."

Assets belonging to the company, Nippon Steel Corp., have been seized and are being put through a process to be liquidated, an outcome Tokyo has warned would do irreparable damage to relations with Seoul.

One condition Japan had set for Suga's participation in the trilateral summit, which would have also been attended by South Korean President Moon Jae In and Chinese Premier Li Keqiang, was a guarantee that the assets would not be sold off.

Suga took office in mid-September after his predecessor, Shinzo Abe, stepped down for health reasons. By continuing Abe's hard-line stance against South Korea, Suga could be seeking to appease the ruling Liberal Democratic Party's conservative base.

In their first phone call, Suga told Moon that bilateral relations are in a "very difficult situation" and called on Seoul to take steps to "return to a constructive relationship" with Tokyo, according to the Japanese government.

But South Korea is unlikely to make such a promise, given Moon's administration has said it cannot overturn a decision by the judicial branch due to the separation of powers. If the process moves ahead, an order to liquidate the assets could be issued as early as December.

Under such conditions, sources from both the Japanese and South Korean governments said the trilateral summit will likely not take place this year.

A spokesman for South Korea's Foreign Ministry indicated Tuesday that the country is still trying to host the summit by year-end, telling reporters that the country is "in consultation with the countries concerned."

Leaders from the three countries began meeting regularly on the sidelines of international conferences in 1999 and have been holding standalone summits since 2008 with rotating hosting duties. The previous two rounds were held in Chengdu, western China in December last year and in Tokyo in May 2018.

Since taking its current form, the summit has been canceled or postponed four times -- in 2013 and 2014 over differing views of wartime history and territorial disputes, in 2016 amid political turmoil in South Korea, and in 2017 due to a scheduling conflict with China.