North Korea has fired three missiles into the sea, the US military says.
Two of the short-range ballistic missiles, which were fired from a site in Gangwon, flew about 155 miles, officials in South Korea said.
The US Pacific Command said that the second missile appears to have blown up immediately.
South Korea's presidential office and military said North Korea fired "several" projectiles in what was presumed to be a test of its 300-millimetre rocket artillery system.
The missile test has prompted South Korea to press ahead with military tests aimed as the US Pacific territory of Guam.
The presidential office in Seoul said the US and South Korean militaries will proceed with their war games "even more thoroughly" in response to the launch.
They are the first known missile firings since July, when the North successfully flight tested a pair of intercontinental ballistic missiles that analysts say could reach deep into the US mainland when perfected.
The White House said that President Donald Trump, who has warned that he would unleash "fire and fury" if the North continued its threats, was briefed on the latest North Korean activity and said "we are monitoring the situation".
Saturday's launch came during an annual joint military exercise between the United States and South Korea that the North condemns as an invasion rehearsal, and weeks after Pyongyang threatened to fire missiles toward Guam.
MORE ABOUT