Pyongyang has long had missiles that can reach targets across South Korea and Japan but is accelerating efforts to develop an intercontinental ballistic missile (ICBM) capable of delivering a nuclear warhead to the continental United States - something President Donald Trump has vowed "won't happen".
May 23 China urged North Korea not to violate U.N. Security Council resolutions with its nuclear and missile programmes, after Pyongyang said it had successfully tested what it called an intermediate-range ballistic missile. "This is the [the Democratic People's Republic of Korea's] answer to the Trump administration".
The solid-fuel Pukguksong-2 missile flew about 500 kilometers (310 miles) and reached a height of 560 kilometers (350 miles) on Sunday before plunging into the Pacific Ocean.
The North conducted two nuclear tests past year.
North Korea continues its testing of ballistic missiles, some of which it claims is capable of carrying a nuclear warhead, despite several warnings and strong sanctions being imposed by the United Nations.
The North's official KCNA news agency, citing the spokesman for the foreign ministry, said the country had "substantially displayed" the capabilities for mounting a nuclear attack on Hawaii and Alaska and had built full capabilities for attacking the USA mainland.
Under the watch of third-generation dictator Kim Jong Un, North Korea has been aggressively pursuing a decades-long goal of putting a nuclear warhead on an intercontinental ballistic missile capable of reaching the USA mainland.
A joint chiefs statement said, "Our military is closely monitoring signs for additional provocation by the North Korean military and we are keeping a full military readiness".
The second known firing of the Pukguksong-2 on Sunday appeared to confirm that trend.More news: Isco's away goal hands CL final berth to Real Madrid
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If North Korea did indeed fire the Pukguksong-2 again, it might be part of attempts to stabilize the system before operationally deploying the missiles, said Kim Dong-yub, an analyst at Seoul's Institute for Far Eastern Studies.
And Sunday's test comes just days after the US Navy said the aircraft carrier USS Ronald Reagan would soon be off the Korean Peninsula, where the carrier USS Carl Vinson is now operating.
If the missile launch is confirmed it would be Pyongyang's eighth missile test this year.
The letters on the top left reads "North Korea, missile launch".
In Tokyo, Japanese Prime Minister Shinzo Abe called the launch a "challenge to the world" that tramples worldwide efforts to resolve the North Korean nuclear and missile problems peacefully.
The footage of the launch was released as China called for dialogue to ease tensions on the peninsular.
"For military purposes, solid-fueled missiles have the advantage that they have the fuel loaded in them and can be launched quickly after they are moved to a launch site", David Wright, co-director of the Union of Concerned Scientists, said in a blog post.
Solid fuel is like an explosive jelly, and less corrosive than liquid fuel, meaning it can be easily stored in the rocket's fuel tank, unlike the liquid alternative, which requires specially lined tanks.