Pyongyang fired two short-range ballistic missiles toward the Sea of Japan, also known as the East Sea, the South Korean military said on Friday.
According to the International Atomic Energy Agency (IAEA), there are also increasing indications of an imminent nuclear test.
South Korean news agency Yonhap cited the Joint Chiefs of Staff as saying that the two missiles were fired overnight from North Korea's northeastern province of Gangwon. No further details were initially released.
United Nations resolutions prohibit North Korea from testing ballistic missiles of any range, some of which are capable of carrying a nuclear warhead.
The situation on the Korean Peninsula is currently very tense. Nuclear-armed North Korea has conducted a string of missile tests in the past few weeks.
The government in Pyongyang has said in its own statements that the tests were intended to simulate the firing of tactical nuclear weapons at South Korean airfields.
Experts have been predicting for some time that North Korean leader Kim Jong Un will perform his first nuclear test since 2017 in the coming weeks.
"We hope it doesn't happen but indications (are) unfortunately going in another direction," IAEA head Rafael Grossi said in New York on Thursday.
"Everybody is holding its breath about this because another nuclear test would be yet another confirmation of a program which is moving full steam ahead in a way that is incredibly concerning," Grossi said.
The latest missile launches came shortly after South Korea's Defense Ministry announced it would begin a major air exercise with U.S. forces next Monday.
The "Vigilant Storm" maneuver will involve 140 aircraft from South Korea, including F-35 stealth fighter jets. The U.S. Air Force plans to mobilize 100 aircraft for the exercise.
North Korea regularly accuses the United States of preparing an attack through its maneuvers with South Korea, which both countries deny. The U.S. has 28,500 soldiers stationed in South Korea.
In view of the threat posed by North Korea's missile and nuclear weapons program, Japan is considering acquiring cruise missiles from its protecting power, the U.S., according to a media report.
The government is considering the purchase of sea-based U.S. cruise missiles of the Tomahawk type, the Japanese news agency Kyodo reported on Friday, citing government sources.
In addition to the threat posed by North Korea, there are concerns around China's growing military ambitions in the region. With cruise missiles, Japan would have the possibility to put enemy missile bases out of action.
The acquisition of such offensive weapons is controversial in Japan, given its pacifist post-war constitution. Japan's security policy has so far been exclusively defense-oriented.
Against the background of the increasingly tense security situation, however, the G-7 country now wants to drastically strengthen the country's defense, Prime Minister Fumio Kishida explained.
A revision of the national security strategy is due at the end of the year. According to Kyodo, the government aims to include the possession of offensive weapons such as cruise missiles.
