North Korea has fired a ballistic missile in the first such test since Donald Trump took office as US president.
Mr Trump assured Japan’s Prime Minister Shinzo Abe that “America stands behind Japan, its great ally, 100%”.
The missile fired by North Korea flew east towards the Sea of Japan for about 500km, South Korean officials say.
Mr Abe said the test was “absolutely intolerable”. Japanese officials say the missile did not reach its waters.
Speaking at a joint press conference during a visit to the US, Mr Abe added that Mr Trump had also assured him that he was committed to “further enforcing our alliance”.
North Korea has conducted a number of nuclear tests in the past year in acts of aggression that continue to alarm and anger the region.
Sunday’s launch took place at 07:55 local time (22:55 GMT Saturday) from the Banghyon air base in North Pyongan province on the west side of the Korean peninsula.
The missile reached an altitude of about 550km (350 miles), according to a South Korean official quoted by Reuters news agency, and appeared to be a Rodong medium-range missile.