North Korea over a three-day period beginning on Saturday fired off what appeared to be six short-range missiles, Reuters reported.
The series of launches included one on Monday morning and another later in the day into waters along the east coast of North Korea, according to the South Korean Defense Ministry.
The North has stepped up its threats and provocations since being hit with new U.N. Security Council sanctions for its February nuclear test.
"We are conducting intense military exercises to strengthen our defense capacity," the Committee for the Peaceful Reunification of Korea said in a Monday Korean Central News Agency report. "Our military is conducting these exercises in order to cope with the mounting war measures from the U.S. and South Korea, which is the legitimate right of any sovereign country."
Three similar weapons were launched on Saturday and one on Sunday, Agence France-Presse reported.
It was not yet sure that the objects fired in recent days were missiles or other weaponry, South Korean officials indicated. Seoul first described the systems fired on Saturday as guided missiles, but later called them "projectiles."
“There is a possibility that they are short-range missiles or large-caliber rockets with a similar ballistic trajectory,” according to Defense Ministry spokesman Kim Min-seok.
The North's last round of short-range missile tests occurred in March, the Wall Street Journal reported.
"We are very worried over North Korea's provocative action," Reuters quoted U.N. Secretary General Ban Ki-moon as saying on Sunday.
"I hope North Korea will refrain from further such actions," he added. "They must soon return to talks and lower tensions."
Meanwhile, U.S. Defense Secretary Chuck Hagel on Friday said leadership of about 28,500 troops in South Korea would be transferred from Gen. James Thurman to Army Lt. Gen. Curtis Scaparrotti, assuming the Senate approves the Afghanistan veteran, AFP reported.