North Korea - if left unchecked - is on an "inevitable" path to obtaining a nuclear-armed missile capable of striking the United States, a senior US defence official says.

The remarks by Defence Intelligence Agency Director Lieutenant-General Vincent Stewart at a Senate hearing on Tuesday are the latest indication of mounting US concern over Pyongyang's advancing missile and nuclear weapons programmes, which the North says are needed for self-defence.

US lawmakers pressed Stewart and Director of National Intelligence Dan Coats to estimate how far away North Korea was from obtaining an intercontinental ballistic missile (ICBM) that could reach the United States.

They repeatedly declined to offer an estimate, saying doing so would reveal US knowledge about North Korea's capabilities, but Stewart warned the panel the risk was growing.

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"If left on its current trajectory the regime will ultimately succeed in fielding a nuclear-armed missile capable of threatening the United States homeland," Stewart said.

"While nearly impossible to predict when this capability will be operational, the North Korean regime is committed and is on a pathway where this capability is inevitable."

The UN Security Council was to meet on Tuesday behind closed doors to discuss Sunday's test of a solid-fuel Pukguksong-2 missile, which defies Security Council resolutions and sanctions. The meeting was called at the request of the United States, Japan and South Korea.

John Schilling, a missile expert contributing to Washington's 38 North think-tank, estimated it would take until at least 2020 for North Korea to be able to develop an ICBM capable of reaching the US mainland and until 2025 for one powered by solid fuel.

But Coats acknowledged gaps in US intelligence about North Korea and the thinking of its leader Kim Jong-un.

He cited technological factors complicating US intelligence gathering, including gaps in surveillance and reconnaissance (ISR), which rely on assets such as spy satellites and drone aircraft.

"We do not have constant, consistent ISR capabilities and so there are gaps, and the North Koreans know about these," Coats said.

Washington has been trying to persuade China to agree to new sanctions on North Korea, which has conducted dozens of missile firings and tested two nuclear bombs since the start of last year.

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Last month, US President Donald Trump called North Korean leader Kim Jong-un a "madman with nuclear weapons" during a telephone call with Philippine President Rodrigo Duterte, according to a transcript of a April 29 conversation released by US media on Tuesday.

"We can't let a madman with nuclear weapons let on the loose like that. We have a lot of firepower, more than he has, times 20. But we don't want to use it," Trump said, citing "two nuclear submarines" the Pentagon sent to the area.

Transcribed by the Philippine government, the conversation was released by The Washington Post and The Intercept.

Trump also queried Duterte about whether he believed Kim was "stable or not stable". The Philippine leader responded their North Korean counterpart's "mind is not working and he might just go crazy one moment".

Kim has a "dangerous toy in his hands that could create so much agony and suffering for all mankind", Duterte added.

But Trump appeared reassured that North Korea's recent missile tests had failed, saying "all his rockets are crashing. That's the good news".

Turning to China and its ability to counter the nuclear threat, Trump pressed Duterte to call Chinese President Xi Jinping to exert pressure.

"I hope China solves the problem. They really have the means because a great degree of their stuff come through China," Trump said. "But if China doesn't do it, we will do it."

Duterte agreed. However, he cautioned: "The other option is a nuclear blast, which is not good for everybody."