SEOUL, South Korea (AP) — North Korea said Thursday it successfully launched ballistic missiles from a train for the first time and was continuing to bolster its defenses, after the two Koreas test-fired missiles hours apart in dueling displays of military might.

Wednesday’s launches underscored a return of the tensions between the rivals amid a prolonged stalemate in U.S.-led talks aimed at stripping North Korea of its nuclear weapons program.

Pyongyang’s official Korean Central News Agency said the missiles were launched during a drill of a “railway-borne missile regiment” that transported the weapons system along rail tracks in the country’s mountainous central region and accurately struck a sea target 800 kilometers (500 miles) away.

State media showed what appeared to be two different missiles streaking up from rail-car launchers engulfed in orange flames along tracks surrounded by dense forest.

A rail-based ballistic system reflects North Korea’s efforts to diversify its launch options, which now includes various vehicles and ground launch pads and may eventually include submarines. Firing a missile from a train could add mobility, but some experts say North Korea’s simple rail networks running through its relatively small territory would be quickly destroyed by enemies during a crisis.

“Our military assesses that North Korea is continuously developing various mobile launch equipment,” said Col. Kim Jun-rak, a spokesman for South Korea’s Joint Chiefs of Staff. He said the South Korean and U.S. militaries were continuing to examine the North’s launches.

The South Korean and Japanese militaries said earlier that North Korea’s two short-range ballistic missiles landed inside Japan’s exclusive economic zone but outside its territorial waters. The last time a North Korean missile landed inside that zone was in October 2019.

Pak Jong Chon, a senior North Korean official who has been seen as influential in the country’s missile development, said Wednesday’s tests were successfully conducted in line with the “strategic and tactical design and intention” of the North’s ruling Workers’ Party.

North Korean leader Kim Jong Un vowed at a party congress in January to bolster his nuclear deterrent in the face of U.S.-led sanctions and pressure and issued a long wish list of sophisticated weaponry, including longer-range intercontinental ballistic missiles, nuclear-powered submarines, spy satellites and tactical nuclear arms.

In another weapons display over the weekend, the North said it tested new cruise missiles, which it intends to make nuclear-capable, that can strike targets 1,500 kilometers (932 miles) away, a distance putting all of Japan and U.S. military installations there within reach.

Hours after the latest North Korean launches, South Korea reported its first test of a submarine-launched ballistic missile. As President Moon Jae-in and other top officials watched, the missile flew from a submarine and hit a designated target, Moon’s office said.

Kim Yo Jong, the powerful sister of the North Korean leader, threatened a “complete destruction” of bilateral relations over Moon’s comments while he observed the test, when he said the South’s growing conventional missile capacities would be a “sure deterrence” against North Korean provocation.

South Korea, which doesn’t have nuclear weapons and instead is protected by the U.S.’s, has been accelerating efforts to build up its conventional arms, including developing more powerful missiles. Observers say Moon’s government, which has been actively pursuing reconciliation with North Korea, may have wanted to appear tougher in response to criticism that it’s too soft on the North.

Kim Yo Jong took offense to Moon describing North Korean weapons demonstrations as a provocation and said warned of dire consequences in inter-Korean relations if he continues on with what she described as slander of North Korea.

Kim Dong-yub, a professor at the University of North Korean Studies in Seoul, said the North Korean photos indicated the rail-fired missiles were a solid-fuel, short-range weapon the North first tested from truck launchers in 2019. The missiles, likely modeled on Russia’s Iskander missiles, are designed to fly at relatively low altitudes where the air is dense enough to allow for maneuverability in flight, making interception by missile defense systems more difficult.

While the North is trying to broaden its launch systems, the analyst Kim questioned whether rail-mobile missiles would meaningfully improve the country’s military capabilities when the North’s simple rail networks would be easy targets during crisis.

Experts say North Korea is building up its weapons systems to apply pressure on the United States in the hopes of winning relief from economic sanctions aimed at forcing the North to abandon its nuclear arsenal. U.S.-led talks on the issue have been stalled for more than two years.

Kim Jong Un’s government has so far rejected the Biden administration’s overtures for dialogue, demanding that Washington abandon what it calls “hostile” policies first — a reference to the sanctions.

The United States said it had no hostile intent and called for North Korea to return to talks. “What we seek to do is to reduce the threat to the United States, to our allies in the region, … and we think we can do that through diplomacy,” State Department spokesman Ned Price told reporters in Washington.

While testing various short-range weapons recently, North Korea has maintained its self-imposed moratorium on nuclear and long-range missile tests, a sign it may not want to scuttle chances for diplomacy entirely.