Meeting for the first time in about four years, the top diplomats from South Korea, Japan and China agreed on Sunday to revive co-operation among the Asian neighbours and resume their leaders’ trilateral summit — but without a specific timing.

Closely linked economically and culturally with one another, the three countries together account for about 25% of the global gross domestic product.

But efforts to boost co-operation have often hit a snag because of a mix of issues including historical disputes stemming from Japan’s wartime aggression and the strategic competition between China and the United States.

“We three ministers agreed to restore and normalise three-nation co-operation at an early date,” South Korean Foreign Minister Park Jin told reporters after his meeting with Japan’s Yoko Kamikawa and China’s Wang Yi in Busan, South Korea.

Mr Park said the three ministers affirmed an earlier agreement by lower-level officials to restart the summit “at the earliest mutually convenient time” and agreed to expedite preparations for the meeting.

Ms Kamikawa separately said the ministers agreed to speed up their work to achieve the summit “at an early and appropriate timing”.

The three also agreed to push for diverse co-operation projects in areas such as people-to-people exchange, trade, technology, public health, sustainable development and security, according to South Korean and Japanese statements.

The lack of an agreement on the timing for the trilateral summit would suggest the top-level gathering will not likely happen this year as South Korea, the chair of the next summit, had hoped, observers say.

In a separate meeting with Mr Wang on Saturday, Ms Kamikawa said she renewed Japan’s demand that China remove its ban on seafood imports from Japan in response to Tokyo’s discharge of treated radioactive wastewater from its tsunami-hit nuclear power plant.

Mr Wang, for his part, said China opposed Japan’s “irresponsible action” of releasing the wastewater and called for an independent monitoring mechanism of the process, according to the Chinese foreign ministry.