North Korea fired two short-range ballistic missiles into the East Sea on Sunday, just three days after it raised military tension by launching three short-range rockets into the sea, Seoul’s Joint Chiefs of Staff said.

“North Korea fired what appeared to be Scud missiles at 4:50 a.m. and 4:58 a.m. from its eastern coastal city of Wonsan, in the North’s Gangwon Province,” the JCS said. “The missiles are analyzed as having a range of about 500 km.”

Stressing that the South Korean military was strengthening its readiness posture to prepare against additional provocations, the JCS added that the firing came again without any prior declaration of a no-fly/no-sail zone.

“The missiles fell into the open sea before reaching Japan’s air defense zone,” said a JCS official, declining to be named. “We believe that the missiles are Scud-C missiles, the same type the North fired on March 3.”

The North has Scud-B missiles with a range of 300 km; Scud-C missiles with a range of 500 km; Scud-D missiles with a range of more than 700 km; and Scud-ER, an upgraded version of Scud-D missiles.

A Pyongyang official told Japan’s Kyodo News that the missile launch was part of a routine military exercise.

The launch of the missiles came just four days before Chinese President Xi Jinping visits South Korea for summit talks with President Park Geun-hye.

Some analysts said that the North might have expressed its displeasure over Xi’s state visit to Seoul. The North Korean ruler and Xi have yet to meet for a summit, while Park and Xi have already met four times.

Analysts also say that the launch appears to be designed to probe South Korea and the U.S. to see how they would respond to the provocation.

“Particularly the fact that the North made the surprise launches early in the morning indicates that it wanted to check the responses from the South Korea-U.S. alliance,” said a military official, requesting anonymity.

“From early this year, the North fired a variety of projectiles, which we believe the North showed off its possession of various means to strike the South.”

Cheong Seong-chang, a senior fellow at the Sejong Institute speculated that the North fired the missiles in response to the recent military drills in the East Sea by the South Korean military, and to strengthen internal unity in its military.

Pyongyang fired short-range projectiles a total of 11 times this year. Most of the launches took place in February and March in an angry response to the South Korea-U.S. military exercise, which the North argues is a rehearsal for a “nuclear war of invasion.”

By Song Sang-ho (sshluck@heraldcorp.com)