The short-range missile flew for six minutes before landing in the Sea of Japan, the US Pacific Command said, adding it was working on a more detailed assessment.
The launch comes in fresh defiance of tough talk from US President Donald Trump, who promised on Friday that the "big problem" of North Korea "will be solved".
Trump’s comments came at the G7 summit in a meeting with Prime Minister Shinzo Abe of Japan, which along with South Korea is most immediately threatened by North Korean belligerence.
Abe swiftly condemned Monday’s launch and vowed "concrete action" with the US.
"We will never tolerate North Korea’s continued provocations that ignore repeated warnings by the international community," Abe told reporters.
"As agreed during the G7 summit, the North Korean problem is the international community’s top priority. In order to deter North Korea, we will take concrete action with the United States," he said.
In Washington, a National Security Council spokesman said Trump had been briefed on the launch.
South Korea’s new president, Moon Jae-In, ordered a meeting of the national security council to assess the launch, which came a day after North Korea said leader Kim Jong-Un had overseen a test of a new anti-aircraft weapons system.
The North Korea has been stepping up efforts to develop an intercontinental ballistic missile that can deliver a nuclear warhead to the continental United States.
The missile launches, and Pyongyang’s threat to stage its sixth nuclear test, have prompted calls for tougher UN sanctions and a warning from Trump that military intervention was an option under consideration.
South Korea’s Joint Chiefs of Staff said Monday’s missile had a flight range of about 450km.
Japan’s chief cabinet secretary, Yoshihide Suga, told reporters the missile appeared to have fallen into the country’s exclusive economic zone (EEZ) -- waters extending 200 nautical miles from its coast.
The regime has carried out two atomic tests and dozens of missile launches since the beginning of last year, with multiple sets of UN sanctions failing to halt its weapons push.
Following North Korea’s test-firing of what analysts said was its longest-range rocket yet earlier this month, the UN Security Council vowed to push all countries to tighten sanctions against Pyongyang.
But China has made it clear that the push for diplomatic talks -- not imposing more sanctions -- was the priority.
The United States has said it is willing to enter into talks with North Korea -- but only if it halts its missile and nuclear tests.
Kim Jong-Un has sought to ramp up North Korea’s nuclear programme under his rule, saying the country needs atomic weapons to defend itself against the threat of invasion.