South Korea and Japan have begun sharing military intelligence on North Korea under a trilateral agreement with the United States, a U.S. defense official said Wednesday.

In December, the three countries signed a memorandum of understanding that calls for voluntary sharing of military secrets on North Korea's nuclear and missile programs between the three countries.

The deal paved the way for Seoul and Tokyo to share such intelligence via the U.S. after the two countries failed to strike a bilateral intelligence sharing deal in 2012 due in part to negative public sentiment in South Korea about signing such a pact with the former colonial ruler.

Since then, the three countries have worked out a detailed formula for intelligence sharing.

"I can confirm that we have begun implementation in accordance with the trilateral agreement," the Pentagon official told Yonhap News Agency on condition of anonymity.

It was the first confirmation that Seoul and Tokyo have shared military secrets under the deal.

Citing "the sensitive nature of the information," however, the official declined to provide any further specifics, including when the first intelligence sharing happened and how many pieces and what type of intelligence have been traded so far.

The intelligence sharing deal is considered an effort to get around frayed relations between South Korea and Japan and move trilateral security cooperation forward.

Relations between South Korea and Japan have been strained for years due mainly to Tokyo's attempts to whitewash its wartime atrocities and colonial occupation. Their relations worsened further after Japanese Prime Minister Shinzo Abe came into office as he took a series of nationalistic steps.

Their ties have shown signs of improvement recently.

On Monday, Abe and South Korean President Park Geun-hye visited each other's embassy to celebrate the 50th anniversary of normalization of diplomatic relations. Seoul's foreign minister, Yun Byung-se, also visited Tokyo this past weekend for the first time since taking office.

The U.S. has called for Seoul and Tokyo to come to terms with each other. Frayed relations between the two key Asian allies are cause for concern for Washington as it seeks to develop three-way security cooperation in an effort to keep a rising China in check. (Yonhap)