Japan lodged a protest with South Korea after flags hoisted during an Olympic preparation match were found bearing disputed islets in the Sea of Japan, Chief Cabinet Secretary Yoshihide Suga said Monday.

"We cannot accept the flag in light of Japan's stance over the sovereignty of Takeshima and it is extremely regrettable," Suga said at a news conference, referring to the South Korean-controlled, Japanese-claimed islands called Dokdo in Korean.

The sports flag appeared during a game between Sweden and the unified Korean women's ice hockey team in Incheon ahead of the Pyeongchang Winter Games.

Suga said Tokyo will continue to urge Seoul to take "appropriate measures" over the issue, which has long been a source of diplomatic friction between the neighbors.

Last month, Seoul demanded the Japanese government close a new museum exhibiting items that back Tokyo's claim to the islets. The government rejected the call.

South Korea has stationed security personnel there and constructed lodgings, a monitoring facility, a lighthouse, and port and docking facilities.

After North Korean leader Kim Jong Un struck a conciliatory tone on ties with the South in his New Year's address, the two Koreas agreed to form a joint women's ice hockey team and march together under a unified flag at the Olympics' opening ceremony on Friday.