South Korea's Supreme Court on Tuesday upheld a lower court ruling from several years ago that ordered a Japanese steel firm to compensate four South Koreans who were victims of forced labor during Japanese colonial rule.

The ruling against Nippon Steel & Sumitomo Metal Corp. is likely to aggravate bilateral ties, already strained over a territorial dispute and the issue of women forced to work at wartime Japanese brothels.

(Labor group members and police officers square off near the Japanese Consulate General in Busan, South Korea, on May 1, 2018, over the erection of a statue symbolizing Korean forced workers taken to Japan during its 1910-1945 colonial rule)

The Japanese government, maintaining its position that the issue of compensation was resolved under a 1965 bilateral accord, quickly condemned the ruling, with Prime Minister Shinzo Abe pledging a "firm" response.

Foreign Minister Taro Kono, who said in a statement that the ruling would overturn the legal basis on which the two countries have built "friendly and cooperative relationship," summoned the South Korean ambassador in Tokyo to lodge a formal diplomatic protest.

South Korean Prime Minister Lee Nak Yeon said he will work with experts to come up with the government's measures on the result. He added that South Korea hopes to improve ties with Japan.

In dismissing the appeal lodged by Nippon Steel & Sumitomo Metal, the top court said the right of individuals to claim damages was not terminated by the accord, which stipulates that issues relating to property and claims between the two countries and their peoples have been settled "completely and finally."

It ordered the Japanese firm to pay 400 million won ($350,000) in compensation to each plaintiff, only one of whom is still alive, as the Seoul High Court had ordered in its July 2013 ruling.

The four men claimed they were deprived of their human rights when they were forced to work at a steel mill that belonged to Japan Iron & Steel Co., which was later known as Nippon Steel Corp. until it merged with another steelmaker in 2012 to form Nippon Steel & Sumitomo Metal.

Nippon Steel & Sumitomo Metal contended that the issue of compensation had been settled under the bilateral agreement, which was attached to the 1965 treaty that established diplomatic ties between Japan and South Korea. The Korean Peninsula was colonized by Japan from 1910 to 1945.

Lee Chun Sik, 94, the sole living plaintiff, told reporters after the ruling, "I feel so heartbroken to be the only one to see the final ruling."

The company issued a statement calling the ruling "extremely regrettable," adding that it will respond "properly" after examining the contents of the ruling and taking the Japanese government's response into account.

The top court's decision represents the first final ruling in South Korea concerning a compensation order against a Japanese company emanating from a post-World War II compensation lawsuit.

Among 14 other similar lawsuits against Japanese companies, the defendants are expected to lose in 11 of them in which orders of compensation were already issued at lower courts.

Among the other Japanese firms that have lost their cases in lower courts in South Korea are Mitsubishi Heavy Industries Ltd. and Hitachi Zosen Corp.

Some of the four plaintiffs originally filed a damages suit with a court in Osaka but lost in 2003. Two years later, they filed a similar suit against Nippon Steel & Sumitomo Metal in South Korea.

They lost the first two court rulings, with the courts citing the previous rulings against Korean forced laborers in Japan and the expiration of the statute of limitations.

However, in 2012 the Supreme Court reversed the previous rulings and ordered a retrial, ruling that their right to seek compensation was not invalidated by the 1965 treaty.