SEOUL, South Korea — A missile exploded in mid-flight, live artillery was fired off two flashpoint islands in the Yellow Sea and balloons crossed the heavily armed border once again as North Korea and South Korea ramped up their escalating war of nerves Wednesday.

Both Koreas are fully engaged in heated bilateral competition: Russian President Vladimir Putin’s high-profile visit to North Korea last week poured further fuel on the fire, and South Korea responded by saying it might send arms to Ukraine if Moscow steps up its military support of Pyongyang.

South Korea’s Joint Chiefs of Staff said that a North Korean missile, possibly a solid-fueled hypersonic missile, exploded in mid-air over the Sea of Japan after it was launched from a location in the east of the capital Pyongyang early Wednesday morning. Heavy smoke trails suggested possible engine issues, the South Korean reports stated, but the missile’s characteristics suggest that North Korean leader Kim Jong-un remains determined to upgrade his arsenal despite warnings from the U.S. and its allies in the region.

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Solid-fuel missiles are more difficult for adversaries to strike preemptively, as they don’t need to be erected and fueled before launch. Hypersonic missiles also pose a problem for air defense systems due to both their high speeds and their unpredictable flight paths.

Pyongyang’s launch followed angry accounts in the North Korean state media criticizing the arrival of the U.S. aircraft carrier USS Theodore Roosevelt in the port of Busan, South Korea as “provocative.” South Korean President Yoon Suk-yeol visited the vessel on Tuesday.  The carrier is set to take part in a newly inaugurated set of trilateral drills with Japan and South Korea, dubbed “Freedom Edge” that were agreed upon in trilateral consultations in early June.

For its part, South Korea for the first time in seven years on Wednesday resumed live-fire artillery drills off the islands of Baengnyeong-do and Yeonpyeong-do. Marines on the islands fired howitzer shells, multiple launch rocket systems and even anti-tank missiles off the coast.

The two Yellow Sea islands are closer to North Korea’s coast than South Korea’s and lie over lucrative crab fishing grounds. In years past, they were the site of the deadliest hostilities between the two states over the years.

In yet another escalation, North Korea on Tuesday released its latest barrage of trash-carrying balloons —  the sixth — across the border into South Korea. While no damage was reported from the balloons, air traffic was suspended at the country’s major airport, Incheon International, for three hours in the early hours of Wednesday morning.

The latest wave of balloons is carrying trash, but prior waves included manure, which, according to South Korean disclosures, contained parasites.

North Korea initiated its southward balloon offensive in retaliation against a smaller number of balloons, loaded with anti-regime propaganda materials, launched northward by activist groups from the South.

Reactions by state media make clear that Pyongyang, which has erected a formidable information wall around its citizenry, is infuriated by the southern balloons.

In response to the “filth balloons,” South Korea, last weekend, briefly restarted the broadcast of programs — news, weather reports and K-pop songs — into North Korea. The broadcasts, delivered by mobile units of camouflaged, military trucks equipped with high-volume speakers, were the first such transmissions in six years. South Korea’s top military officials say they are mulling resuming a wider information offensive, depending on  North Korea’s actions.

But the ongoing incidents, particularly the balloon intrusions, showcase the vulnerability of South Korea’s capital area, which lies in the northwestern part of the country just 30 miles south of the DMZ. Incheon International is approximately 25 miles southwest of the frontier, and Gimpo International, in northern Seoul, is just 15 miles from the North’s guns across the border.

Pyongyang, by contrast, is about 100 miles north of the tense and heavily armed border and a much harder target to hit.

“We have no strategic depth,” said Moon Chung-in, who teaches at Seoul’s Yonsei University and once advised the liberal administrations that engaged and negotiated with North Korea.