People watch a TV news program reporting on North Korea's artillery shells at Seoul Railway Station in Seoul, South Korea, on Monday. North Korea fired artillery shells into waters near the two countries’ sea border, Seoul's military said. (Lee Jin-Man/AP)

North Korean leader Kim Jong Un flexed his military muscles again Monday as his country fired more than 100 rockets and artillery shells into the sea near the border with South Korea, just a day after firing two ballistic missiles over the peninsula.

Kim has steadily been ratcheting up tensions with his neighbors since he took over from his father, Kim Jong Il, 21 / 2 years ago, leading to perceptions in Washington that he is both erratic and weak.

While his father tended to keep his powder dry for times when he really needed to lodge a protest, the younger Kim has presided over an unusually large number of missile and artillery tests this year alone, according to South Korean officials.

On Monday, North Korea fired about 100 artillery rounds into the Sea of Japan, also known as the East Sea, from a site on its east coast, just a few hundred yards from the demilitarized zone that separates the two Koreas.

Some shells traveled as far as 30 miles and landed within one mile of the Northern Limit Line, the countries’ de facto maritime border, but did not cross into southern waters, said Um Hyo-sik, a spokesman for South Korea’s Joint Chiefs of Staff. The shelling appeared to be part of a military drill, he said.

“It is not unusual for Pyongyang to carry out such a shelling on its east coast, but it is rare that the North has done that near the military demarcation line,” a military officer told the South’s Yonhap news agency.

It was the latest in a series of similar incidents in recent weeks. On Sunday, North Korea fired two ballistic missiles from the border city of Kaesong, on the western side of the peninsula, into the eastern sea. Kim reportedly attended the launch.

The displays are thought to be a protest against joint U.S.-South Korean military exercises set to start this week. The North views such exercises as provocative.

The aircraft carrier USS George Washington arrived in the South Korean port city of Busan on Friday, triggering an angry — but not unusual — denunciation from the North of “gunboat diplomacy.”

“The U.S. should properly understand that the more persistently it resorts to reckless nuclear blackmail and threat, the further [North Korea] will bolster up its cutting-edge nuclear force for self-defense,” said a spokesman for the North’s National Defense Commission, according to the official Korean Central News Agency.

The exercises anger the North not only because it has to scramble its relatively poorly equipped military in response, but also because Pyongyang thinks such large-scale drills are unwarranted during a period without nuclear tests, according to Americans who have recently met with North Korean representatives.

North Korea last set off a nuclear device — its third test — in February 2013. Predictions of a fourth test have not come true.

While the joint exercises are often thought to be the trigger for North Korean rocket launches, Narushige Michishita, a regional security expert at the National Graduate Institute for Policy Studies in Tokyo, offered a different theory.

“My guess is that Kim Jong Un is trying to test the effectiveness of his armed forces,” he said. “By having this unit do this and that unit do that, he might be trying to consolidate his control over his armed forces and find out which military units are capable and which are not.”

Regardless, the Kim regime is sending mixed messages to its neighbor and archrival.

In a surprising gesture, the North said last week that it would send cheerleaders along with about 150 athletes to the Asian Games set to be held in the South in September. The two sides are due to hold discussions Thursday about the event.