South Korea on Monday put Japan into its own new export category, intensifying a trade war between the two neighbours and US allies.
The move came after Seoul announced earlier this month it would remove Tokyo from its list of trusted trading partners, reciprocating an identical move by Japan.
That followed Tokyo's imposition of tough restrictions on exports crucial to tech titans such as Samsung following a series of South Korean court rulings ordering Japanese firms to pay for forced labour during World War II.
The dispute has raised concerns over the potential implications for their security cooperation in the face of North Korean missile tests, and the possible impact on global supply chains.
South Korea's list of trade partners is currently divided into two groups, those who are members of the world's top four export control agreements and those who are not.
But Seoul's trade ministry said Monday it added a new category for countries that had signed the four pacts "but operate an export control system that violates international norms".
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Japan is the only country in the new category.
"Since it's hard to work closely with a country that frequently violates the basic rules... we need an export control system that addresses this," South Korean trade minister Sung Yun-mo told reporters.
Sung did not offer examples of such violations by Japan.
The revision will be implemented in September, he said, adding that Seoul was open to negotiations with Tokyo.
Under the new regulations, South Korean firms must submit five documents -- from the current three -- to win approvals for exporting sensitive items to Japan, with the process taking up to 15 days.