North Korea has test-fired two ballistic missiles, the latest in a series of rocket launches.
US officials said the medium-range missiles, launched off the east coast, flew about 800km (500 miles) before falling into the water.
Afterwards, the US called for Pyongyang to refrain from raising tensions.
It comes a day after US President Barack Obama imposed new sanctions, following North Korea's "illicit" nuclear test and satellite launch.
His executive order freezes North Korean government property in the United States. It bans US exports to - or investment in - North Korea and also greatly expands powers to blacklist anyone, including non-Americans, dealing with North Korea.
The 6 January nuclear test and 7 February satellite launch were violations of existing UN sanctions, as are ballistic missile tests.
US defence officials said two launches had been tracked of what appeared to be Nodong missiles fired from road-mobile launch vehicles.
With a maximum range of 1,300km, the Nodong would have the capability to reach all of South Korea and parts of Japan. North Korea's last Nodong test was in March 2014.
Japanese PM Shinzo Abe condemned the launch and said his government would work with the US and South Korea to coordinate a response.
Lt Col Michelle Baldanza, from the US defence press office said: "We call on North Korea to refrain from actions that further raise tensions in the region."
Amid the heightened tensions, the North sentenced a US student to 15 years hard labour on Wednesday for "severe crimes" against the state.
The US demanded North Korea immediately release Otto Warmbier, 21, who was arrested for trying to steal a propaganda sign from a hotel while on a visit in January.
The US and South Korea are also holding their biggest annual military drills this month, which routinely generate tension.
But this year North Korea threatened to launch a "pre-emptive nuclear strike of justice" against the US and South Korea.