North Korea launched multiple ballistic missiles toward the sea on Sunday, its neighbors said, days after the U.N.’s nuclear watchdog warned that North Korea was making “very serious” advances in efforts to build nuclear weapons.South Korea’s Joint Chiefs of Staff said the launches happened on Sunday morning from the Sinpo area, an eastern coastal site where North Korea has a major shipyard used for building submarines. South Korea's military said it has bolstered its surveillance posture and is closely exchanging information with the U.S. and Japan.Video above: Suicide Drones in North Korea? The Country Is Working on Mass Producing ThemSouth Korea’s presidential office said its National Security Council plans to hold an emergency meeting to discuss the launches.Japan’s Defense Ministry also detected the launches, saying the weapons were believed to have landed in the waters off North Korea’s east coast. It said Tokyo strongly protested to Pyongyang, saying Sunday’s launches threaten regional and international peace and violated U.N. Security Council resolutions that bans any ballistic activities by North Korea.The launches were the latest in North Korea's run of weapons testing activities this year.Last week, North Korea said leader Kim Jong Un supervised missile tests from the country’s destroyer. In the previous week, North Korea said it had three days of testing activities to examine ballistic missiles armed with cluster-bomb warheads and other new weapons systems. Last month, it said it tested an upgraded solid-fuel engine for missiles capable of reaching the U.S. mainland.Kim has focused on enlarging his nuclear and missile arsenals since his high-stakes nuclear diplomacy with U.S. President Donald Trump collapsed in 2019. Trump has repeatedly expressed his desire to restore diplomacy with Kim, and the North Korean leader has recently left open the door for dialogue with Trump but urged Washington to drop demands for the North’s nuclear disarmament as a precondition for talks.On Wednesday, International Atomic Energy Agency Director General Rafael Grossi said that his agency has confirmed “a rapid increase” in activities at nuclear manufacturing facilities in North Korea. Grossi told reporters in Seoul that activities in North Korea point to “a very serious increase” in its nuclear weapons production capabilities.His comments echoed a view by many outside observers that North Korea has taken steps to expand its main Yongbyon nuclear complex and build additional uranium-enrichment sites in recent years. Last September, South Korea’s Unification Minister Chung Dong-young said that North Korea was operating four uranium enrichment facilities and that they were running everyday. Yamaguchi reported from Tokyo.