Japanese residents have been urged to take shelter after South Korea's military said North Korea fired an unidentified missile on Friday from its capital Pyongyang that flew over Japan before landing in the northern Pacific Ocean.
US Defense Secretary Jim Mattis told reporters that top US officials have fully coordinated after the test-launch. "We have just got done with the calls we always make to coordinate among ourselves. Steady as she goes," Mattis said.
It was the second aggressive test-flight over the territory of the close US ally in less than a month and it followed the sixth and most powerful nuclear test by North Korea to date on 3rd September.
South Korea's Joint Chiefs of Staff said the missile travelled about 3,700 kilometres (2,300 miles) while reaching a maximum height of 770 kilometres (478 miles).
The missile was launched from Sunan, the site of Pyongyang's international airport.
North Korea last month used the airport to fire a Hwasong-12 intermediate range missile that flew over northern Japan.
The North then declared it a "meaningful prelude" to containing the US Pacific island territory of Guam and the start of more ballistic missile launches towards the Pacific Ocean.
Japan's Chief Cabinet Secretary Yoshihide Suga denounced North Korea's latest launch, saying he was conveying "strong anger" on behalf of the Japanese people.
Suga said Japan "will not tolerate the repeated and excessive provocations."
The North American Aerospace Defense Command determined the intermediate range missile did not pose a threat to North America. In addition, US Pacific Command said it did not pose a threat to Guam.
South Korean experts said the August launch was Pyongyang's attempt to make missiles flying over Japan an accepted norm as it seeks to test new missiles and win more military space in the region dominated by its enemies.
The Offices of Guam Homeland Security and Civil Defense said the latest launch posed no immediate threat to Guam or the Marianas.
South Korea's Defense Ministry said the country's military conducted a live-fire drill of a Hyunmoo-2 ballistic missile in response to the North's launch on Friday.
Seoul's presidential office said President Moon Jae-in has scheduled a National Security Council meeting to discuss the latest launch.
North Korea claimed its latest nuclear test was a detonation of a thermonuclear weapon built for its developmental intercontinental ballistic missiles.
The North flight-tested its Hwasong-14 ICBMs twice in July and analysts say the missiles potentially could reach deep into the US mainland when perfected.
North Korea initially flight-tested the Hwasong-12 and Hwasong-14 missiles at highly lofted angles to reduce range and avoid neighbouring countries.
The two launches over Japan indicate North Korea is moving towards conducting launches at angles close to operational as it seeks to test whether the warheads survive the harsh conditions of atmospheric re-entry and detonate properly.
North Korea has been accelerating its nuclear weapons development under leader Kim Jong Un, a third-generation dictator in his 30s, who has conducted four of North Korea's six nuclear tests since taking power in 2011.
His military has maintained a torrid pace in testing weapons, which also include solid-fuel missiles designed to be launched from road mobile launchers or submarines.
In his aggressive pursuit of nuclear weapons, Kim is seen as seeking a real nuclear deterrent to help ensure the survival of his government and also the stronger bargaining power that would come from it.
North Korea's latest missile launch came days after the UN Security Council unanimously approved new sanctions over the nuclear test. The latest sanctions are not as tough as what the US had sought, they are expected to have a significant impact.