The United States on Monday welcomed the rare celebrations that South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe attended to mark half a century of relations, saying it looks forward to the relations getting "broader and deeper."

Earlier in the day, Park and Abe visited each other's embassy in Seoul and Tokyo to celebrate the 50th anniversary of the normalization of diplomatic relations between the two neighboring nations.

During the visits, the two leaders called for improving the long-strained relations, with Park saying she hopes this year will be the first year of opening up a new future together and Abe saying he hopes the two countries will work hand in hand to chart a new era of relations.

Though the celebrations fell short of a direct meeting between Park and Abe, they underlined the willingness of both sides to improve their relations, which have been frayed due mainly to historical tensions stemming from Japan's 1910-45 colonial rule of Korea.

"We certainly welcome their agreement to participate in those events. ... We more broadly welcome efforts to improve the bilateral relationship between Japan and Korea," State Department spokesman John Kirby said at a regular press briefing.

"I think it's an important step that they've agreed to attend this commemoration together. That President Park is willing to go and do that, I think that's not insignificant," he said. "And certainly, should that lead to better relations, better cooperation, better dialogue between the two, that's always welcome too. ... Certainly we look forward to that relationship getting broader and deeper."

In a separate statement, a department spokesman also said that the U.S. congratulates the two countries on their 50th anniversary of relations and welcomes efforts by Park and Abe "to commemorate the anniversary in a positive spirit."

"We believe that strong and constructive relations between countries in the region promote peace and stability and are in their interests and in the interests of the United States," the official said.

Relations between Seoul and Tokyo have been badly strained for years, due mainly to Tokyo's attempts to whitewash its wartime atrocities and colonial occupation, including Japan's sexual enslavement of Korean and other Asian women for its troops during World War II.

Frayed relations between the two allies have been a key cause for concern for the U.S. as it seeks to bolster three-way security cooperation, seen as a key pillar of U.S. President Barack Obama's "pivot to Asia" initiative aimed in part at keeping a rising China in check. (Yonhap)