A New Year’s Day speech by Kim Jong-un, the North Korean leader, contained a dramatic shift in tone and policy regarding the South. After ignoring South Korea for years, Mr. Kim called for urgent dialogue to discuss improving ties and easing military tensions on the divided Korean Peninsula, even as he claimed an ability to strike the mainland United States with nuclear missiles.

Mr. Kim also agreed to a request by President Moon Jae-in of South Korea to send a North Korean delegation to the Winter Olympics to be held in the South next month.

Mr. Kim’s about-face, which was broadcast on state-run television, came just days after Washington rallied its allies and rivals to support increasingly punishing United Nations sanctions against North Korea.