North Korea has test-launched its first intercontinental ballistic missile, a potential game-changing development in what may be the world's most dangerous nuclear stand-off.
It is a direct rebuke to US President Donald Trump's earlier declaration that such a test "won't happen!" and appears to be North Korea's most successful missile launch yet.
A US scientist examining the height and distance said the missile could potentially be powerful enough to reach Alaska.
US secretary of state Rex Tillerson later confirmed it was indeed an intercontinental ballistic missile, calling it a "new escalation of the threat" to the US.
He said: "Global action is required to stop a global threat. Any country that hosts North Korean guest workers, provides any economic or military benefits, or fails to fully implement UN Security Council resolutions is aiding and abetting a dangerous regime."
In a direct response to the missile launch, US and South Korean soldiers fired "deep strike" precision missiles into South Korean territorial waters.
The US Eighth Army said the missile firings were a show of force meant to demonstrate US-South Korean solidarity.
US Ambassador to the United Nations Nikki Haley asked for an emergency meeting of the Security Council, which will happen later on Wednesday.
North Korea's Academy of Defence Science said the test of an ICBM - the Hwasong-14 - marked the "final step" in creating a "confident and powerful nuclear state that can strike anywhere on Earth".
It will be difficult to confirm many details about what happened as North Korea's weapons programme is perhaps the most closely held state secret in one of the world's most suspicious nations.
North Korea has previously launched satellites in what critics said were disguised tests of its long-range missile technology.
A test-launch of an ICBM, however, would be a major step in developing nuclear-armed missiles that could reach anywhere in the United States.
The launch sends a political warning to Washington and its chief Asian allies, Seoul and Tokyo, while also allowing North Korean scientists a chance to perfect their still-incomplete nuclear missile programme.
It came on the eve of the US Independence Day holiday, days after the first face-to-face meeting of the leaders of South Korea and the United States, and before a global summit of the world's richest economies.
US, South Korean and Japanese officials say it flew for about 40 minutes and reached an altitude of 1,500 miles, which would be longer and higher than any similar North Korean test previously reported.
It also covered a distance of about 580 miles.
US missile scientist David Wright estimated the highly lofted missile, if the reported time and distance are correct, could have a possible maximum range of 4,160 miles, which could put Alaska in its range.
Hamish de Bretton-Gordon, a former commanding officer of the British Armed Forces Joint Chemical Biological Radiological Nuclear Regiment, said: "In capability of missile terms and delivery, it is a major step up and they seem to be making progress week-on-week."
He added, however, that "actually marrying the warhead to the missile is probably the biggest challenge, which they appear not to have progressed on".
Soon after the morning launch, President Trump responded on Twitter: "North Korea has just launched another missile. Does this guy have anything better to do with his life?
"Hard to believe that South Korea and Japan will put up with this much longer. Perhaps China will put a heavy move on North Korea and end this nonsense once and for all!"
"This guy" presumably refers to North Korean leader Kim Jong Un. China is North Korea's economic lifeline and only major ally, and the Trump administration is pushing Beijing to do more to push the North toward disarmament.
After North Korea claimed earlier this year it was close to an ICBM test launch, Mr Trump tweeted: "It won't happen!"
China and Russia proposed on Tuesday that North Korea declare a moratorium on nuclear and missile tests while the US and South Korea refrain from large-scale joint military exercises.
North Korea views the exercises as preparation for an invasion and has repeatedly demanded their cancellation.
It says it needs nuclear weapons and powerful missiles to cope with what it calls rising US military threats.
The Korean Peninsula has been divided since before the 1950-53 Korean War. Almost 30,000 US troops are stationed in South Korea.