The country launched four banned ballistic missiles overnight
The United States has confirmed it is taking steps to enhance its ability to defend against North Korea's ballistic missiles.
Donald Trump's press secretary says America is deploying an advanced anti-missile defence system to South Korea.
North Korea launched four ballistic missiles into the sea off Japan overnight.
While North Korean state media says the launches were training for an attack on a US base in Japan.
North Korean news agency KCNA said the launches were an exercise designed to prepare the country for a strike.
Leader Kim Jong-Un had overseen the launch, the agency said, and had lauded what he had witnessed.
KCNA said the exercise was carried out by an artillery unit "tasked to strike the bases of the US imperialist aggressor forces in Japan in contingency".
The four missiles flew an average of 1,000 km (620 miles) and reached an altitude of 260 km (160 miles), Japan's Defence Minister Tomomi Inada said.
Some landed as close as 300 km (190 miles) from Japan's northwest coast.
South Korea's Office of Joint Chiefs of Staff said in a statement: "South Korea and the United States are conducting a close-up analysis, regarding further information."
Foreign Affairs Minister Charlie Flanagan called it "an unacceptable act".
Minister Flanagan said earlier: "It is another flagrant violation of North Korea’s obligations under international law, and I urge North Korea to refrain from any further provocative actions. I call on North Korea to re-engage in credible and meaningful dialogue with the international community."
The US and Japan have called for the United Nations Security Council to meet over the launches, which is expected to take place on Wednesday.