North Korea has fired a ballistic missile in the first such test since Donald Trump took office as US president.
The missile fell in seas between the Korean Peninsula and Japan, South Korean officials said.
There was no immediate confirmation from the North, which had recently warned it is ready to test its first intercontinental ballistic missile.
The reports come days before the North is to mark the birthday of leader Kim Jong Un's late father, Kim Jong Il.
Mr Trump assured Japan's Prime Minister Shinzo Abe that "America stands behind Japan, its great ally, 100%".
Mr Abe said the test was "absolutely intolerable". Japanese officials say the missile did not reach its waters.
In a ballroom at Mr Trump's south Florida estate, Mr Abe read a brief statement in which he called on North Korea to comply fully with relevant UN Security Council resolutions.
South Korea said the test launch, along with leader Kim Jong Un's threat to fire an intercontinental ballistic missile in his New Year's address, shows the "irrational nature" of a government that "fanatically" obsesses with developing nuclear ballistic missiles.
South's Foreign Ministry strongly condemned the launch as a "blatant and obvious" violation of UN Security Council resolutions and a "serious threat" to international security.
The ministry says that the South will continue to work with allies including the US, Japan and the European Union to ensure a thorough implementation of sanctions against the North and make the country realise that it will "never be able to survive" without discarding all of its nuclear and missile programs.
Italy warned that North Korea's "repeated missile tests" threaten peace and that the country must stop challenging the international community.
In a statement, the Italian foreign ministry expressed "great concern" about the news that the North had launched a ballistic missile.
It said the repeated tests, combined with North Korea's development of a nuclear arsenal, "constitute a threat to peace and to international security".
Italy said North Korea "must abandon the development" of such an arsenal and "interrupt its path of challenging the international community and of self-isolation."
The US Strategic Command said it had detected and tracked what it assessed was a medium- or intermediate-range ballistic missile.
The command said the launch occurred near the north-western city of Kusong.
Kim Jong Un said in his New Year's address that the country has reached the final stages of readiness to test an ICBM, which would be a major step forward in its efforts to build a credible nuclear threat to the United States.
Though Pyongyang has been relatively quiet about the transfer of power to the Trump administration, its state media has repeatedly called for Washington to abandon its "hostile policy" and vowed to continue its nuclear and missile development programmes until the US changes its diplomatic approach.
Just days ago, it also reaffirmed its plan to conduct more space launches, which it staunchly defends, but which have been criticised because they involve dual use technology that can be transferred to improve missiles.
