The US says a missile fired by North Korea into the Sea of Japan was an intercontinental ballistic missile.
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 described it as 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.
Just hours after the North’s test, the US asked for an urgent meeting of the UN Security Council to discuss the issue. A close-door session of the 15-member body is expected on 5 July.
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.
North Korea, it said, was now “a full-fledged nuclear power that has been possessed of the most powerful inter-continental ballistic rocket capable of hitting any part of the world”.
The launch, the latest in a series of tests, was in defiance of a ban by the UN Security Council.
But experts also believe that Pyongyang does not have the capacity to miniaturise a nuclear warhead that can fit onto such a missile.
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.
Despite this technical achievement, however, it is likely many outside North Korea will continue to be sceptical. They will ask for proof of working guidance, re-entry vehicle, and even a nuclear warhead.
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.
What is an ICBM?
- A long-range missile usually designed to carry a nuclear warhead
- The minimum range is 5,500km (3,400 miles), although most fly about 10,000km or more
- Pyongyang has previously displayed two types of ICBMs: the KN-08, with a range of 11,500km, and the KN-14, with a range of 10,000km, but before 4 July had not claimed to have flight tested an ICBM. It is not clear what differentiates the Hwasong-14
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.
US President Donald Trump also responded swiftly on Tuesday.
On his Twitter account he made apparent reference to North Korean leader Kim Jong-un, saying: “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!”
President Trump has repeatedly called on China, Pyongyang’s closest economic ally, to pressure North Korea to end its nuclear and missile programmes.
On the prospect of North Korea being able to strike the US, he tweeted in January: “It won’t happen.” However experts say it might – within five years or less.
Meanwhile, UK Foreign Secretary Boris Johnson said the international community “must redouble its efforts to impose a price on this regime, which strains every nerve and sinew to build nuclear weapons and launch illegal missiles, even as the people of North Korea endure starvation and poverty”.