WASHINGTON (Yonhap News) -- The U.S. government formally requested Tuesday that South Korea and Japan maintain “restraint” in resolving their latest diplomatic tensions over Dokdo, a set of Seoul-controlled islets claimed by Tokyo.

The appeal came after Japan’s publication of a new defense white paper describing Dokdo as its territory. Koreans were already upset over an attempt by three right-wing Japanese lawmakers to make an inspection tour of Ulleung Island, which serves as an administrative base for Dokdo in the East Sea, earlier this week.

They were denied entry into South Korea and turned back at a Seoul airport.

South Korean officials pledged a strong response to Japan’s renewed territorial claims, a legacy of its imperialist past. In protest, President Lee Myung-bak is reportedly considering canceling a visit to Japan scheduled later this year.

“It’s an issue that’s been handled with restraint. We would hope that such restraint would continue to be exercised and that they would work -- both South Korea and Japan -- work peacefully and diplomatically to find a mutually acceptable solution,” State Department spokesman Mark Toner said at a press briefing.

He emphasized that his government does not take a position with regard to the sovereignty over Dokdo.

South Korea remains firm that Dokdo is its territory historically, geographically and by international law. South Korea does not want it to become an international issue.

Japan’s territorial ambition is one of the biggest obstacles to further development in the neighboring nations’ relations, officials say.