Within hours of being elected South Korea’s president, Lee Jae-myung began​ work on Wednesday by calling for dialogue with its arch enemy, North Korea, to bring peace to the Korean Peninsula, while stressing South Korea’s commitment to its security alliance with the United States.

In his nationally televised inauguration speech at the rotunda lobby hall of the National Assembly, Mr. Lee reaffirmed diplomatic cooperation with ​the Trump administration, with whom he must work with to negotiate over tariffs and maintain its security alliance, and pledged to solidify trilateral cooperation with the United States and Japan.

But Mr. Lee shied away from commenting directly on how he would tackle the thorny diplomatic challenges he faces as president, such as the growing rivalry between the United States, South Korea’s only military ally, and China, its largest trading partner.

Unlike his campaign-trail speeches, where ​Mr. Lee called for improving ties with​ China, his swearing-in address did not mention ​it by name​, reflecting the delicate diplomatic negotiations his government will face in the coming months. Mr. Lee wants to mend his country’s strained relations with China to help spur economic growth. But Washington is asking Seoul to play a bigger role in containing China.

​On Wednesday, Mr. Lee ​only made an indirect reference to China: “I will approach relations with neighboring countries from a perspective of national interest and pragmatism​.”