North Korea on Tuesday launched what appeared to be its longest-range ballistic missile yet, with experts suggesting it could reach Alaska, triggering a Twitter outburst from US President Donald Trump who urged China to “end this nonsense once and for all”.

If the test — which came as the United States prepared to mark its independence day on the Fourth of July — represents an intercontinental ballistic missile (ICBM) it would force a recalculation of the strategic threat posed by Pyongyang.

The North has long had ambitions to build a rocket capable of delivering an atomic warhead to the continental United States — something that Trump has vowed “won't happen”.

It said it would make an “important announcement” later Tuesday, the South's Yonhap news agency reported.

Analysts say the isolated, impoverished country has made great progress in its missile capabilities in the years since the ascension to power of young leader Kim Jong-Un, who has overseen three nuclear tests and multiple rocket launches.

In response to the latest, Trump asked on Twitter: “Does this guy have anything better to do with his life?”

There are still doubts whether the North can miniaturise a nuclear weapon sufficiently to fit it onto a missile nose cone, or has mastered the technology needed for it to survive re-entry into the Earth's atmosphere.

But Tuesday's launch was the latest in a series of provocations that have ratcheted up tensions, and came days after Seoul's new leader Moon Jae-In and Trump focused on risks from Pyongyang in their first summit.

The United Nations has imposed multiple sets of sanctions on Pyongyang over its weapons programmes, which retorts that it needs nuclear arms to defend itself against the threat of invasion.

'All of Alaska'

The “unidentified ballistic missile” was fired from a site in North Phyongan province, the South's Joint Chiefs of Staff said in a statement, and came down in the East Sea, the Korean name for the Sea of Japan.

It flew for “more than 930 kilometres”, they added.

US Pacific Command confirmed the test and said it was a land-based, intermediate range missile that flew for 37 minutes, adding the launch did not pose a threat to North America.

It was estimated to have reached an altitude of more than 2,500 kilometres, Japan said.

“That's it. It's an ICBM,” responded arms control specialist Jeffrey Lewis on Twitter. “An ICBM that can hit Anchorage not San Francisco, but still.”

David Wright, of the Union of Concerned Scientists, wrote on the organisation's allthingsnuclear blog that the available figures implied the missile “could reach a maximum range of roughly 6,700 km on a standard trajectory”.

“That range would not be enough to reach the lower 48 states or the large islands of Hawaii, but would allow it to reach all of Alaska.”

Growing threat

The device came down in the Sea of Japan within the country's exclusive economic zone, the defence ministry said in a statement, waters extending 200 nautical miles from its coast.

Prime Minister Shinzo Abe told reporters: “This launch clearly shows that the threat has grown.” The US, Japan and South Korea will hold a summit on the sidelines of this week's G20 meeting on the issue, he added.

“Also I will encourage President Xi Jinping and President Putin to take more constructive measures.”

South Korea's President Moon, who backs both engagement with the North to bring it to the negotiating table and sanctions, strongly condemned what he called an “irresponsible provocation”.

North Korea's nuclear and missile programmes “threaten the lives of our people and the security of our allies”, he told a meeting of the South's National Security Council.

Independence Day

Washington, South Korea's security guarantor, has more than 28,000 troops in the country to defend it from its Communist neighbour, and fears of conflict reached a peak earlier this year as the Trump administration suggested military action was an option under consideration.

There has also been anger in the United States after Otto Warmbier, an American student detained in North Korea on a tourist trip around 18 months ago, was returned home in a coma in June, dying days later.

Trump has been pinning his hopes on China — North Korea's main diplomatic ally — to bring pressure to bear on Pyongyang.

Last week he declared that Beijing's efforts had failed, but returned to the idea on Twitter following the launch: “Perhaps China will put a heavy move on North Korea and end this nonsense once and for all!” But a former foreign policy adviser to Hillary Clinton warned that his comments risked undermining the credibility of both the US deterrent, and its assurances to its allies in Seoul and Tokyo.

She added: “Picking a twitter fight with a nuclear-armed dictator is not wise — this is not reality TV anymore.”

Trump and China's Xi struck a friendly tone when they met at the US leader's Florida resort in Florida in April, but he infuriated Beijing last week by approving a $1.3 billion arms sales to self-governing Taiwan.

The US administration also imposed sanctions on a Chinese bank accused of laundering North Korean cash, voiced concern about freedom in semi-autonomous Hong Kong and placed the country on a list of the world's worst human trafficking offenders.

Shea Cotton, a researcher at the James Martin Center for Nonproliferation Studies in the US, suggested the North Korean launch was deliberately timed to coincide with the anniversary of the US declaration of independence.

“It's already 4th of July in North Korea,” he said on Twitter. “I somewhat suspect they're shooting off some fireworks today specifically because of that.“