North Korea has fired two projectiles into the sea, South Korea's military has said.
Seoul's Joint Chiefs of Staff said they were launched from an area near its neighbour's east coast, the country's semi-official Yonhap news agency reported.
Based on initial information it appeared to be similar to previous short-range missiles fired by Pyongyang, a US official said, speaking on the condition of anonymity.
Another senior US administration official added: "We are aware of reports of a missile launch from North Korea, and we continue to monitor the situation. We are consulting closely with our Japanese and South Korean allies."
The latest launch comes shortly after Donald Trump said he had received a "very beautiful letter" from North Korean leader Kim Jong-un.
The secretive communist state has fired a series of missiles and rockets since the leaders agreed at a June 30 meeting to revive stalled denuclearisation talks.
Mr Trump played down the recent North Korean weapons launches when he spoke to reporters earlier on Friday, saying: "I say it again: There have been no nuclear tests. The missile tests have all been short-range. No ballistic missile tests. No long-range missiles."
Claiming he could have another meeting with Mr Kim the US president said his counterpart was "not happy" about the missile tests and joint military exercises conducted by the US and South Korean militaries this month.
The largely computer-simulated drills were conducted as an alternative to previous large-scale annual operations that were halted to expedite denuclearisation talks.
North Korea decries such exercises as a rehearsal for war aimed at toppling its leadership.
The pariah state described recent test-firings as a new rocket artillery system and short-range ballistic missile launches.
It previously has called them a "solemn warning" to South Korea over its plans to continue military drills with the United States.
