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 flew east towards the Sea of Japan for about 500km (300 miles), 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 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".
During his election campaign, Mr Trump said US defence commitments to Japan and South Korea were unfair and also called for Japan to pay the full cost of stationing US troops on its soil.
Read more: BBC News