South Korea's president says he and the leaders of Japan and China have adopted a joint declaration that includes their pledge to work closely together to achieve regional peace and prosperity.

Yoon Suk-yeol made the announcement during a joint news conference with Japanese Prime Minister Kishida Fumio and Chinese Premier Li Qiang, which was held after their talks on Monday.

The summit involving the three countries was held for the first time in nearly four-and-a-half years.

Yoon told reporters that the foundation of cooperation among the three countries is mutual understanding and trust, and their leaders must meet frequently to communicate with each other.

Yoon said the three leaders agreed to make the trade and investment environment transparent and predictable and to establish a secure supply chain.

He said the three countries will also cooperate to tackle environmental and other issues.

Yoon also referred to a plan by North Korea to launch what Pyongyang calls an artificial satellite sometime between May 27 and June 3. He said such a launch is a clear violation of United Nations Security Council resolutions and the international community must respond firmly to it.