The US State Department said Wednesday it is longstanding US policy to use only one name for international waters, after President Donald Trump referred to the body of water between Korea and Japan as the Sea of Japan.

South Korea has long campaigned for dual usage of the name East Sea, arguing Sea of Japan is a legacy of Tokyo's imperialist past, including its 1910-45 colonization of the Korean Peninsula.

Trump was in Japan earlier this week when he mentioned the Sea of Japan as part of a pep talk with American troops stationed in the country.

"We understand that the Republic of Korea uses a different name," a State Department spokesperson told Yonhap on condition of anonymity. "As a matter of longstanding policy and practice, the United States government uses only one name to refer to all high seas features."

The spokesperson went on to explain that the US government uses names decided by the US Board on Geographic Names, and that board's approved name for that body of water is the Sea of Japan.

"We encourage Japan and the Republic of Korea to work together to reach a mutually agreeable way forward on this issue," he added.

Japan registered the name Sea of Japan with the International Hydrographic Organization in the early 1920s, when Korea was under Japan's colonial rule.

For Seoul, the naming issue is especially sensitive because Japan continues to lay claim to South Korea's easternmost islets of Dokdo, which lie in the East Sea.

In response to Trump's comment, South Korea's foreign ministry said Tuesday that the government's consistent position is that the East Sea must be jointly used. (Yonhap)