Jan. 9 (UPI) -- The South Korea-based assets of Japanese firm Nippon Steel are being seized following a South Korean court order to confiscate more than $300,000 of the firm's assets in a joint venture.

The document received at joint venture firm PNR on Wednesday orders the firm to forfeit $356,000 in assets as compensation for its use of Korean forced laborers during wartime, Yonhap reported.

Nippon Steel and South Korean steel manufacturer POSCO established PNR, which under court orders must sell 81,075 shares of stock and transfer the sales proceeds to plaintiffs.

The court order and other issues have escalated tensions between South Korea and Japan, two key U.S. allies in the region.

Controversies including a dispute over an incident involving a Japanese aircraft and a South Korean naval ship that took place in December are being taken seriously in Tokyo.

Katsuyuki Kawai, special adviser on foreign affairs for Japanese Prime Minister Shinzo Abe, said Tuesday South Korean actions are unacceptable.

"Last month in the Sea of Japan a South Korean naval destroyer targeted a maritime self-defense force aircraft with its Signal Tracking and Illuminating Radar," Kawai said, according to NHK and TBS. "The Korean side is taking on action that is inconsistent with those of an ally."

Japan has said the South Korean naval ship targeted the Japanese aircraft with fire-control radar and released a 13-minute video clip of the incident. Seoul has said the claim is false.

Kawai also said the court order on asset seizure is a "strange response" and the United States should make a statement on the issues.

South Korean news service Newsis reported Wednesday Japan has sent a request to Seoul's foreign ministry requesting "diplomatic negotiations" on the court order and other issues.

Seoul said the request is "under review."