North Korea fired two "unidentified projectiles" into the waters between South Korea and Japan on Thursday afternoon, the South Korean government said in a statement.
It is unclear what exactly was launched, but South Korea's Joint Chiefs of Staff said the military "is maintaining its readiness by monitoring relevant movements in preparation for further launches."
Japan's Coast Guard said in a statement "it appears that North Korea has launched a missile" and advised ships to avoid the area.
If confirmed as a missile test, it would be the first by Pyongyang in nearly a month and the 12th since May. The country said it tested a new type of submarine-launched ballistic missile on October 2. Experts voiced concern over that event because it was the first missile test by North Korea in some time that didn't involve a shorter-range weapon.
The US and North Korea held working-level nuclear talks a few days later, but those concluded without an agreement. Both sides offered a very different picture of events -- Pyongyang accusing Washington of lacking flexibility, but the State Department said the US "brought creative ideas and had good discussion with its DPRK counterparts," using the formal acronym for North Korea.
Despite their differences, US President Donald Trump and North Korean leader Kim Jong Un have continued to speak positively of their personal relationship.
Kim was quoted last week as saying his relationship with Trump is "special" and the two leaders maintain "trust in each other."
However, Pyongyang has been particularly critical of Trump's advisers and the diplomats around him. And the clock on striking an agreement may be ticking.
Kim said in an important policy speech in April that he would give the Trump administration until the end of the year to change its negotiating strategy. Since then, North Korea resumed test-firing missiles and has reiterated in state media that Pyongyang is giving the US until the end of the year to solve the issue. It's unclear how serious that deadline is.
This week also marked the first time Kim has communicated with South Korean President Moon Jae-in in weeks.
Kim sent a letter to Moon to offer condolences to the South Korean leader after his mother died, according to Moon's office.
The letter was delivered Wednesday and handed over at Panmunjom, the joint security area in the demilitarized zone that divides North and South Korea.