South Korea has lodged an informal protest against a U.S. government-funded agency's decision to use Japan's name for the body of water between Seoul and Tokyo, a diplomatic source said Monday.

Seoul and Tokyo have tussled for decades over the name of the sea between the Korean Peninsula and Japan. South Korea calls it the "East Sea," while Japan claims the name "Sea of Japan."

The protest by South Korea's foreign ministry came after the U.S. maritime-boundary agency recently notified the International Hydrographic Organization (IHO) that it backed only the usage of "Sea of Japan," according to the source.

South Korea said the name "Sea of Japan" was unfairly established during Japan's 1910-45 colonial rule of the Korean Peninsula, arguing the Japanese appellation is a vestige of Tokyo's imperialistic past that should be changed.

Since officially naming the body of water the East Sea in 1992, South Korea has lobbied the international community to simultaneously use both "East Sea" and "Sea of Japan."

"The U.S. agency's decision unilaterally supports Japan," the source said on the condition of anonymity.

"We conveyed our stance to the U.S. government that East Sea and Sea of Japan should be used simultaneously," the source said, declining to identify the U.S. agency's official name.

In response, the U.S. government played down the agency's decision, saying it was not Washington's official stance over the naming issue, the source said.

About 70 IHO member states are scheduled to meet next year to update the hydrographic information and data in their publication.

South Korea is pushing for the simultaneous use of East Sea and Sea of Japan in the IHO publication.

Diplomatic tensions between South Korea and Japan have recently risen over a renewed Japanese move to strengthen its territorial claim to the Dokdo islets in the East Sea.

Four lawmakers from Japan's conservative opposition Liberal Democratic Party were banned early last week from traveling to a South Korean island close to Dokdo.

South Korea also summoned a senior Japanese embassy official last week and protested Japan's 2011 defense white paper, which renewed its territorial claim to Dokdo. (Yonhap News)