The United States is seeking to establish a formula for implementing a three-way military intelligence sharing deal with South Korea and Japan aimed at better coping with North Korea's nuclear and missile threats, a source familiar with the matter said Thursday.

The issue was a key topic when Defense Secretary Chuck Hagel spoke by phone with South Korean Defense Minister Han Min-koo this week, the source said. Hagel is expected to talk about it again when he speaks with Japanese Defense Minister Gen Nakatani later Thursday.

Last month, 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.

Now that the MOU has been signed, the three countries should "get the ball rolling," the source said, referring to U.S. efforts to establish a formula to carry out the agreement.

The formula is expected to lay out detailed procedures, including how to determine intelligence to be shared. Such procedures are necessary because the three countries have no idea what intelligence the other partners have, the source said.

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.

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 a 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)