SEOUL » North Korea on Wednesday test-fired two medium-range ballistic missiles, South Korea said, a defiant challenge to a meeting by the leaders of rivals South Korea, Japan and the United States that focused on the North’s security threat.

The launch of what are believed to be Rodong missiles would be a violation of U.N. Security Council resolutions and marks a big escalation from a series of shorter-range rocket launches the North has staged in recent weeks to protest ongoing annual military drills by Washington and Seoul that the North claims are invasion preparation.

It would be the North’s first launch of this type of missile since 2009, said South Korean Defense Ministry spokesman Kim Min-seok.

Kim told reporters that that the missiles flew about 403 miles off North Korea’s east coast early Wednesday morning. It wasn’t immediately clear where the missiles splashed down. Kim said the missiles were likely fired from a mobile launcher.

The North’s arsenal of an estimated 300 Rodong missiles could in theory be fitted with nuclear warheads — once Pyongyang masters the ability to miniaturize atomic bombs — and, with a range of up to 800 miles, could reach Tokyo and key U.S. military bases in Japan.

The launch comes on the fourth anniversary of the sinking of a South Korean warship that Seoul and other nations blame on a North Korean torpedo. Pyongyang denies involvement in the attack, which killed 46 sailors. It also poses a big challenge to what had been recently improving relations between Pyongyang and Seoul.

A year after threatening each other with war, the bitter rivals had restored some trust and held reunions of families divided by the Korean War of the early 1950s. The Korean Peninsula remains officially at war because that war ended with an armistice, not a peace treaty.

North Korean state media made no immediate comment on the launch.

Joel Wit, a former State Department official and editor of the 38 North website, said the launch could be a serious setback to recent efforts by North Korea to improve relations with South Korea and Japan. It also could put China, the North’s only major ally, in an awkward position if and when the U.S. seeks further sanctions at the United Nations.

China has shown increasing annoyance with North Korean provocations, but Beijing also wants to avoid shaking Pyongyang and possibly jeopardizing stability along its borders.

North Korea is thought to have a handful of rudimentary nuclear bombs, but most analysts don’t believe Pyongyang has yet mastered the ability to build warheads small enough to mount on a missile that could threaten the United States. To achieve that goal, Pyongyang has staged several long-range rocket tests in recent years and, a year ago, its third nuclear test. Talks aimed at ending the North’s nuclear program, meanwhile, have been stalled since 2009.

The most recent launch came as U.S. President Barack Obama, Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-hye met Tuesday in the Netherlands to discuss North Korea’s security threat. It was Park and Abe’s first face-to-face meeting since they both took office more than a year ago. Many in Asia are angry over Japan’s treatment of historical issues related to World War II and Tokyo’s colonization of the Korean Peninsula in the first half of the 20th century.

A North Korean diplomat on Monday criticized the United States for conducting military exercises near its borders and accused Washington of undermining the prospect of improved relations with South Korea.

Last year, North Korea responded to international condemnation of its third nuclear test and the annual springtime U.S.-South Korean military drills by threatening nuclear strikes on Washington and Seoul. Analysts say the impoverished North chafes against the drills, which Washington and Seoul call routine and defensive in nature, because it has to spend precious resources responding with its own exercises.

The North’s response to the war games this year had been more muted because of what analysts see as a desire by Pyongyang to use improving ties with Seoul to win badly-needed aid and outside investment.