The US has confirmed that North Korea tested a long-range missile which some experts believe could reach Alaska.
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 claimed on Tuesday to have successfully tested an intercontinental ballistic missile (ICBM).
In response the US and South Korea fired missiles in the South’s waters.
The combined military exercise was designed to show their “precision fire capability”, the Pentagon said.
Though Tuesday’s test appeared to be a significant development, experts believe that North Korea does not yet have long-range nuclear weapon capabilities.
Tillerson also 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”.
The US has 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.
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”.
North Korea’s official KCNA news agency later quoted Kim Jong-un as saying the test was a “gift” to the Americans on their independence day.
The launch, the latest in a series of tests, was in defiance of a ban by the UN Security Council.
But most experts believe Pyongyang does not yet have the capability to miniaturise a nuclear warhead that can fit onto a long-range missile, and that such missiles cannot accurately hit targets.
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, BBC 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, reports the BBC.