North Korea fired two unidentified projectiles into the sea off its eastern coast on Saturday, South Korea's military said.
The latest launch comes shortly after US President Donald Trump said he had received a "very beautiful letter" from North Korean leader Kim Jong Un.
North Korea has fired a series of missiles and rockets since Kim and Trump agreed at a June 30 meeting to revive stalled denuclearisation talks.
A US official said that at least one projectile was launched and that it appeared to be similar to previous short-range missiles fired by Pyongyang.
It was the fifth round of missile launches in two weeks, and came just days after US and South Korean forces began their defense readiness exercises, which Kim's government has said are a "flagrant violation" of the diplomatic process between Pyongyang, Washington, and Seoul.
The newest launch came after Trump strained to keep the door open for denuclearisation negotiations with Kim after months in which no progress has been made.
While other US officials have called the launches "provocations," Trump said he agreed with the North Korean leader and was hoping to meet him again, with talks having been frozen for months.
"I got a very beautiful letter from Kim Jong Un yesterday," Trump said. "It was a very positive letter."
"He wasn't happy with the war games," Trump said, referring to the US-South Korean military exercises.
"As you know, I've never liked it either. I've never been a fan. And you know why? I don't like paying for it."
Hours later, the White House had no immediate comment on the newest rocket tests.
"We are aware of reports of a missile launch from North Korea, and we continue to monitor the situation.
But others in the Trump administration sound less enthusiastic about Kim's missiles.
After Pyongyang's fourth launch early this week, Secretary of State Mike Pompeo said Washington was ready to resume talks, but that the target of the "full, final denuclearisation of North Korea" had not changed.
A senior State Department official who refused to be named told journalists last week that the missile tests were an impediment to peace.
"The missile launches, any kind of provocations, are not helpful to advancing the cause of diplomacy," the official said.
The official said there was a unified message going to the North Koreans from China and Russia that they need to "cease the provocations, reengage in diplomacy to achieve complete denuclearisation."
The United States and South Korea have kicked off their largely computer-simulated exercises as an alternative to previous large-scale annual drills that were halted to expedite denuclearisation talks.
North Korea decries such exercises as a rehearsal for war aimed at toppling its leadership.
Large solid-fuel rocket engines for North Korea's ballistic missile program are most likely being produced at a factory complex in Hamhung, monitoring group 38 North said last year. Hamhung also has a testing site for those engines.