North Korea fired a ballistic missile into the sea off its east coast early on Sunday.
It is the first such test since US President Donald Trump was elected, and his administration indicated that Washington would have a calibrated response to avoid escalating tensions.
The test was of a medium- or intermediate-range missile that landed in the Sea of Japan, according to the US defence department, not an Intercontinental Ballistic Missile (ICBM), which the North has said it could test at any time.
The missile was launched from an area called Panghyon in North Korea's western region just before 8 am (2300 GMT Saturday) and flew about 500 kilometres (300 miles), South Korea's Office of the Joint Chiefs of Staff said.
"Our assessment is that it is part of a show of force in response to the new US administration's hardline position against the North," the office said in a statement.
The South's military said Seoul and Washington were analysing the details of the launch. Yonhap news agency said the South Korean military was assessing the launch to confirm whether it was a Musudan intermediate-range ballistic missile, which has a designed range of 3,000 km (1,800 miles).
The latest missile test was conducted as US President Donald Trump was hosting Japanese Prime Minister Shinzo Abe.
Appearing with Trump at a news conference at the president's south Florida estate, Abe condemned the missile launch as "absolutely intolerable."
Trump followed Abe with even fewer words, saying in part, "I just want everybody to understand and fully know that the United States of America stands behind Japan, its great ally, 100 percent."
Abe read a brief statement in which he called on the North to comply fully with relevant UN Security Council resolutions. He said Trump has assured him of US support and that Trump's presence showed the president's determination and commitment.
TRT World spoke to Dina Demetrius in Los Angeles for more details.