North Korea fired a salvo of short-range ballistic missiles early Wednesday (local time), Seoul's military said, according to the AFP news agency.
Seoul's Joint Chiefs of Staff (JCS) said it had "detected and (was) analyzing several short-range ballistic missiles launched to the northeast around 06:50" local time.
"In preparation for additional launches, our military has strengthened monitoring and vigilance, while closely sharing information" with Japan and the US, it added.
Tokyo also confirmed the launch, with the country's coastguard saying one missile had splashed down already.
"Vessels please pay attention to information coming ahead and if you spot fallen objects please don't approach closer but report it to the coastguard," it said in a statement quoted by AFP.
The launch is North Korea’s second this past week. Last Thursday, the North fired what Seoul described as multiple short range ballistic missiles into waters east of the Korean peninsula.
North Korean state media later claimed that this had been a test of a "new-type 600mm multiple rocket launcher" which was overseen by the country’s leader, Kim Jong Un.
North Korea has staged dozens of launches this year, including in July, when it tested a new tactical ballistic missile capable of carrying a 4.5-ton super-large warhead.
In January, the country launched a hypersonic missile. In response, South Korea, the United States and Japan carried out joint naval drills in waters off southern Jeju Island.
North Korea then announced it had tested an "underwater nuclear weapon system" in response to the joint naval exercises.
In April, North Korea successfully test-fired a new mid- to long-range solid-fuel hypersonic missile.
Weeks later, the country fired what appeared to be a ballistic missile which fell into the sea.