Photo/IllutrationThe flag that got Japan's back up is displayed during a joint North and South Korean women's ice hockey team friendly in Incheon, South Korea, on Feb. 4. (Nobuhiro Shirai)

Japan took umbrage over a flag hoisted by the joint North and South Korean women's ice hockey team showing a unified Korean Peninsula with a group of islets at the center of a territorial dispute between Tokyo and Seoul.

Chief Cabinet Secretary Yoshihide Suga said Feb. 5 Tokyo had filed a protest with Seoul through diplomatic channels.

“We cannot accept the flag in light of our country’s position and find the matter extremely regrettable," Suga said. "We are urging South Korea very strongly to make an appropriate response.”

The flag was hoisted during a friendly game in the South Korean city of Incheon on Feb. 4 ahead of the Pyeongchang Olympic Games that kick off Feb. 9.

The Japanese government was irked that the flag includes the Takeshima islets in the Sea of Japan. The islets are claimed by Japan but administered by South Korea, which calls them Dokdo.