Pyongyang has doubled down on upgrading its weapons program this year despite facing crippling economic sanctions.
"North Korea fired an unidentified ballistic missile(s) into the East Sea," Seoul's Joint Chiefs of Staff said Sunday morning, referring to the Sea of Japan.
US and South Korean officials have warned for weeks that Pyongyang may conduct a seventh nuclear test.
That launch came just days after U.S. President Joe Biden left South Korea following a meeting with newly elected President Yoon Seok-youl.
Despite struggling with a recent Covid-19 outbreak, North Korea has resumed construction on a long-dormant nuclear reactor, new satellite imagery has indicated.