The US has confirmed that North Korea on Tuesday tested an intercontinental ballistic missile (ICBM).
Secretary of State Rex Tillerson called it a “new escalation of the threat” to the US and the world and warned that Washington “will never accept a nuclear-armed North Korea”.
Pyongyang earlier said it was its first successful intercontinental ballistic missile test.
US officials believe the North may now be able to fire a missile to Alaska.
However, experts say it cannot accurately hit a target.
In response to the test over the Sea of Japan, the US and South Korea conducted a “combined [military] exercise to show our precision fire capability”, Pentagon spokeswoman Dana White said in a statement.
The US also asked for an urgent meeting of the UN Security Council to discuss the issue. A closed-door session of the 15-member body is expected later on Wednesday.
In a statement, Mr Tillerson said: “The United States strongly condemns North Korea’s launch of an intercontinental ballistic missile.
“Testing an ICBM represents a new escalation of the threat to the United States, our allies and partners, the region, and the world.”
Mr Tillerson stressed that “global action is required to stop a global threat”.
And he warned that any nation that provided economic or military benefits to the North or failed to fully implement UN Security Council resolution was “aiding and abetting a dangerous regime.
What did North Korea say earlier on Tuesday?
The announcement on North Korea state television said the Hwasong-14 intercontinental ballistic missile (ICBM) test was overseen by leader Kim Jong-un.
It said the projectile had reached an altitude of 2,802km (1,731 miles) and flew 933km for 39 minutes before hitting a target in the sea.
How far could this missile travel?
The big question is what range it has, says the BBC’s Steven Evans in Seoul. Could it hit the United States?
David Wright, a physicist with the US-based Union of Concerned Scientists, says that if the reports are correct, this missile could “reach a maximum range of roughly 6,700km on a standard trajectory”.
That range would allow it to reach Alaska, but not the large islands of Hawaii or the other 48 US states, he says.
It is not just a missile that North Korea would need, our correspondent adds. It must also have the ability to protect a warhead as it re-enters the atmosphere, and it is not clear if North Korea can do that.
What does this test tell us? By defence expert Melissa Hanham
Once again North Korea has defied the odds and thumbed its nose at the world in a single missile launch. With the test of the Hwasong-14, it has shown that it can likely reach intercontinental ballistic missile ranges, including putting Alaska at risk.
Kim Jong-un has long expressed his desire for such a test, and to have it on the 4 July holiday in the US is just the icing on his very large cake.
From a technical perspective, though, their engines have demonstrated ICBM ranges, and this would be the first of several paths North Korea has to an ICBM with even greater range.
Are neighbours and nuclear powers concerned?
South Korea’s President Moon Jae-in has called on the United Nations Security Council to take steps against North Korea.
But a strong warning came from the country’s Director of Operations of the Joint Chiefs of Staff. Cho Han-Gya said “Kim Jong Un’s regime will face destruction” if it “ignores our military’s warnings and continues provocations”.
Japan said “repeated provocations like this are absolutely unacceptable” and Prime Minister Shinzo Abe said his country would “unite strongly” with the US and South Korea to put pressure on Pyongyang.