President Xi Jinping told newly elected South Korean President Lee Jae-myung on Wednesday that China attaches "great importance" to strengthening ties.

"Uncertainties are increasing in the international and regional situation," state broadcaster CCTV reported Xi as telling Lee in a congratulatory message.

"I attach great importance to the development of China-South Korea relations," he said.

The Foreign Ministry, for its part, said Beijing has always opposed taking sides and forming camps and is willing to work with Seoul to promote the healthy and stable development of relations.

Lee, who was sworn in earlier in the day following his victory in a snap election on Tuesday, has expressed more conciliatory plans for ties with Beijing and Pyongyang, singling out in particular the importance of China as a major trading partner.

Xi is among a group of neighbours welcoming Lee at a time when US President Donald Trump has thrown the country's traditional allies into disarray with his "unilateral tariffs".

Japanese Prime Minister Shigeru Ishiba said he wanted to "energise" cooperation, with the two neighbours becoming closer under Lee's disgraced predecessor Yoon Suk-yeol, and that included through three-way talks with joint ally the United States.

"This year marks the 60th anniversary of the normalisation of diplomatic relations between Japan and Korea," Ishiba said in Tokyo.

"With President Lee Jae-myung, I would like to energise cooperation between Japan and South Korea as well as between Japan, the United States and South Korea."

Ishiba added that "it would be good if we could hold a Japan-South Korea summit in the near future".

US Secretary of State Marco Rubio congratulated Lee and voiced hope for working with the left-leaning leader who previously has sought greater distance from the United States.

"The United States and the Republic of Korea share an ironclad commitment to the alliance grounded in our Mutual Defense Treaty, shared values and deep economic ties," Rubio said. (Reuters/AFP)