North Korea launched several projectiles as a test on Saturday morning, Seoul's Joint Chiefs of Staff (JCS) said.
The JCS are investigating the details of the launch.
An unidentified projectile believed to be a short-range missile was fired at about 9:00 a.m. local time (12:00 a.m. UTC) from near the east coast city of Wonsan towards the Sea of Japan.
It was followed by several others, which flew 70-200 kilometers (44-124 miles).
If confirmed, it would mark the country's first documented missile launch since it fired an intercontinental ballistic missile (ICBM) in November 2017. After that launch it declared its nuclear capabilities complete and restarted talks with the US and South Korea.
White House Press Secretary Sarah Sanders said: "We are aware of North Korea's actions tonight. We will continue to monitor as necessary."
Japan's Defense Ministry said the projectiles fell well short of the country's coast and that Japan is not facing any security threat.
On Friday, South Korean Foreign Minister Kang Kyung-wha said Pyongyang should show "visible, concrete and substantial" denuclearization action if it wanted sanctions relief.
Talks between North Korean leader Kim Jong Un and US President Donald Trump collapsed in February over the terms of potential sanctions relief and the nations have been at loggerheads since.
Earlier in the week, North Korean Vice Foreign Minister Choe Son Hui warned of an "unwanted outcome" if Washington did not adjust its stance on economic sanctions.
aw/bw (Reuters, dpa, AFP, AP)