1 of 2 | North Korea launched a suspected hypersonic missile early Wednesday morning but it exploded in mid-flight over the East Sea, South Korea’s Joint Chiefs of Staff said. Photo by Yonhap

A JCS official later told reporters that the missile flew around 155 miles and exploded in the air off the east coast of the country, spreading debris into the sea over a radius of several miles.

The official, who spoke under the condition of anonymity, speculated that the North was attempting to refine the development of a solid-fuel hypersonic missile it had test-fired earlier this year.

More smoke than usual appeared to emanate from the missile, suggesting the possibility of combustion issues, the official added.

In April, North Korea claimed it successfully launched a new solid-fuel intermediate-range hypersonic missile, a nuclear-capable weapon with a range that could place U.S. military installations in Guam — around 2,100 miles away — within reach.

Such a weapon represents challenges for missile detection and interception systems. Missiles using solid-fuel propellants can be transported and launched more quickly than liquid-fuel models, while hypersonic weapons travel at least five times the speed of sound and are maneuverable mid-flight.

The launch comes amid international concerns over a mutual defense treaty signed by North Korea and Russia during Russian President Vladimir Putin’s visit to Pyongyang last week.

It also follows North Korea’s condemnation of a port call by an American nuclear-powered aircraft carrier to the South Korean city of Busan.

In a statement carried by state-run Korean Central News Agency on Monday, the North’s Vice Defense Minister Kim Kang Il called the arrival of the USS Theodore Roosevelt a “provocative action” and warned that North Korea may respond by “demonstrating its overwhelming and new deterrent force.”

North Korea sent trash-filled balloons over the border with the South for a second day in a row, the JCS also said Wednesday, continuing back-and-forth provocations that have raised tensions near the demilitarized zone that separates the two Koreas.

The North launched more than 250 balloons carrying scrap paper overnight, the JCS said in a message to reporters, with around 100 landing in Seoul and the northern part of neighboring Gyeonggi Province.

The latest launch marked the sixth time in the last month that North Korea has sent balloons filled with debris and even excrement, according to South Korean officials.

South Korea briefly resumed anti-Pyongyang loudspeaker broadcasts at the border earlier this month in response to the launches. On Tuesday, the military said it was prepared to begin the transmissions again at any time.

The North’s balloon launches appear to be a response to the longstanding practice of North Korean defectors floating balloons with anti-Pyongyang messages across the border.

Activist group Fighters for a Free North Korea said it sent 20 balloons carrying some 300,000 leaflets, USB drives containing South Korean media and U.S. dollars across the border last week.

The DMZ has been the site of multiple border incursions in recent weeks, with North Korean troops crossing the military demarcation line three times since June 9.

On each occasion, the South Korean military fired warning shots and the North’s soldiers returned to their side of the border.

JCS officials said the crossings appear unintentional, as the North has been ramping up construction activity in frontline areas of the DMZ since withdrawing from an inter-Korean military agreement in November.