North Korea fired two short-range ballistic missiles off its east coast on Wednesday, the South's military said, just a day before US Vice President Kamala Harris is set to arrive in Seoul.
The launch came two days after South Korea and U.S. forces conducted a military drill in waters off the South's east coast involving an aircraft carrier. On Sunday, North Korea had fired another ballistic missile towards the sea off its east coast.
Wednesday's missiles were launched from the Sunan area of Pyongyang, the North Korean capital, between 18:10 p.m. (0910 GMT) and 18:20 p.m. (0920 GMT), South Korea's Joint Chiefs of Staff said.
"Our military has strengthened surveillance and vigilance and is maintaining airtight readiness posture while closely cooperating with the United States," they said in a statement.
Japan's coast guard also reported a suspected ballistic missile test, which its minister of state for defence, Toshiro Ino, condemned as "unacceptable".
He told reporters, "North Korea's series of actions, including repeated ballistic missile launches, poses a threat to the peace and security of Japan, the region and international society."
Following a stop in Japan, Harris will land in the South Korean capital and visit the heavily fortified Demilitarised Zone (DMZ) between the neighbours on Thursday.
In a speech hours earlier aboard the destroyer USS Howard destroyer in the Japanese city of Yokosuka, Harris had called Sunday's missile launch part of an "illicit weapons programme which threatens regional stability and violates multiple U.N. Security Council resolutions".
North Korea has tested missiles at an unprecedented pace this year, while this week's joint drill is a show of force intended to warn against what could be its first nuclear test since 2017.
The isolated country has completed preparations for a nuclear test, a window for which could open between China's party congress in October and the U.S. mid-term elections in November, South Korean lawmakers said on Wednesday.
Since 2006, North Korea has been subject to U.N. sanctions, which the Security Council has steadily, and unanimously, stepped up over the years to cut off funding for its nuclear weapons and ballistic missile programmes.
North Korea rejects the U.N. resolutions as an infringement of its sovereign right to self-defence and space exploration, and has criticised military exercises by the United States and South Korea as proof of their hostile intentions.