SEOUL (Reuters) - South Korean members of parliament visited a set of remote islands also claimed by Japan on Tuesday, a move likely to further fray ties between two of the most important U.S. allies in Asia.

The tour followed a visit by South Korean President Lee Myung-bak in August, the first by a Korean leader, which prompted Japan to recall its ambassador to Seoul. The row was then overshadowed by a far more acute dispute between Tokyo and Beijing over another island group.

South Korea controls the islands, known as Dokdo in Korean and Takeshima in Japan and equidistant from the mainlands of both countries.

A Korean police official on the islands said the parliamentarians, flying in by helicopters for the third such visit, were briefed on coast guard and other activities as part of an "audit" by parliament's Defence Committee.

Japan issued a formal protest to South Korea's ambassador.

"We've confirmed that the committee members landed on Takeshima," Japanese Chief Cabinet Secretary Osamu Fujimura told reporters in Tokyo. "Takeshima is sovereign Japanese territory and such a step is completely unacceptable."

Japan, which ruled Korea for more than three decades last century, has said it will take the issue of the rocky island chain to the International Court of Justice.

But South Korea must first give its assent, a notion it rejects as it has long considered the islands its territory "historically, geographically and by international law".

Experts say there is no indication that the dispute would develop into the kind of diplomatic standoff that has severely damaged ties and trade between Tokyo and Beijing, although it remains a major irritant for the United States.

Strained relations between Seoul and Tokyo over issues related to Japan's colonial rule of Korea scuppered a planned intelligence sharing deal earlier this year.

A Korean study said has said there could be deposits of natural gas near the islands, although no exploratory work has been conducted.

(Reporting by Jack Kim and by Anton Slodkowski in Tokyo, Editing by Ron Popeski)