The weapons North Korea launched over the weekend traveled into the stratosphere and flew at a distance long enough to strike deep into South Korean territory, according to a South Korean assessment.

South Korea’s Joint Chiefs of Staff did not indicate what was fired off of North Korea’s east coast on Saturday and into the sea, but experts believe at least one ballistic missile was launched and the testing may indicate that North Korea has improved its capabilities to strike the South.

The multiple short-range projectiles were fired on Saturday from 9:06 a.m. to 10:55 a.m. and reached altitudes of between 20-60 kilometers (12-37 miles). They covered a distance of about 70 to 240 kilometers (45 to 150 miles), Kim Joon-rak, a spokesman for the Joint Chiefs of Staff, told reporters at a briefing.

That means the weapons could strike a region stretching from the Seoul area — accounting for about half of the country’s population — to the central city of Daejeon.

Neither the U.S. nor South Korea have confirmed North Korea fired a ballistic missile, which would be in violation of international agreements and complicate their current detente with Pyongyang. Rather than condemn the move, U.S. and South Korean officials played down the threat, a move analysts said that could keep the door open for nuclear negotiations but also signal to Kim Jong Un that he has the green light to do more short-range testing.

The weapons test indicates that North Korea may be looking to thwart U.S. missile interceptors, according to Kim Ki-ho, a defense professor at Kyonggi University in Seoul and former army colonel.

The U.S. operates a system known as the Terminal High Altitude Area Defense, also called THAAD, which can intercept missiles flying at an altitude of 40 kilometers or more. Low-altitude missiles can be defended by the Patriot missile defense system.

The latest North Korean weapons appeared to fly “too low to be intercepted by the THAAD” and “too fast” to be intercepted by the Patriot system, he said. “The latest tactical guided weapons could incapacitate those system.”

When asked about the interceptor-evasion possibility, South Korea’s defense ministry said separately in a statement that “comprehensive reviews are underway, including such matters.” U.S. Forces Korea did not immediately respond to requests for comments.

Based on a photo Pyongyang released in its state media, a satellite image from Planet Labs Inc. showing what appeared to be a single missile contrail at the exercise site and the missile’s trajectory, North Korea appears to have fired a new solid-fuel ballistic missile similar to a Russian Iskander, said Melissa Hanham, a nonproliferation expert and director of the One Earth Future Foundation’s Datayo Project.

North Korea had unveiled a similar weapon, which could be stored while fueled, deployed and fired with less detection time, during a military parade in February 2018.

The last time North Korea fired a ballistic missile was in November 2017, at the height of tensions with the U.S.