South Korea's top court ruled Tuesday that a Japanese steel producer must compensate South Koreans for their forced labor during World War II.

Japan denounced the verdict against Nippon Steel & Sumitomo Metal Corp. as "unthinkable," while expressing hope that cooperation over North Korea would not be jeopardized.

The landmark ruling saw South Korea's Supreme Court uphold a 2013 order for the company to pay 100 million won ($87,700) to each of the four steel workers who initiated the suit in 2005.

Lee Choon-shik, 94, is the sole surviving plaintiff. He welcomed the ruling, but told a televised news conference that it was "heartbreaking to see it today, left alone alive."

The two countries share a bitter history, including Japan's 1910-45 colonization of the Korean peninsula and the use of "comfort women," Japan's euphemism for girls and women, many of them Korean, forced to work in its wartime brothels.