President Donald Trump believes that communist leader Kim Jong Un will eventually develop better missiles but has said that "we can't allow it to happen".

In a taped interview broadcast yesterday on CBS' Face the Nation, the President would not discuss the possibility of military action after the latest failed North Korean rocket launch, saying: "It is a chess game. I just don't want people to know what my thinking is."

Separately, Trump's national security adviser, Army Lieutenant General HR McMaster, said North Korea's most recent missile test, on Saturday, represents "open defiance of the international community". He said North Korea poses "a grave threat", not just to the US and its Asian allies, but also to China.

Speaking on Fox News, McMaster said it was important "for all of us to confront this regime, this regime that is pursuing the weaponisation of a missile with a nuclear weapon".

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"This is something that we know we cannot tolerate," McMaster said.

On Saturday, a North Korean mid-range ballistic missile broke up a few minutes after launch, the third test-fire flop of last month. The programme's repeated failures over the past few years have given rise to suspicions of US sabotage.

In the CBS interview, the President was asked why the North's rockets keep blowing up.

"I'd rather not discuss it," he said. "But perhaps they're just not very good missiles. But eventually, he'll have good missiles." He added: "And if that happens, we can't allow it to happen." Trump also called North Korea's leader "a pretty smart cookie" for being able to hold onto power after taking over at a young age. "People are saying, 'Is he sane?' I have no idea," the President said.

North Korean ballistic missile tests are banned by the United Nations because they are seen as part of the North's push for a nuclear-tipped weapon that can hit the US mainland.

McMaster said that Trump "has made clear that he is going to resolve this issue one way or the other", but that the President's preference was to work with China and others to resolve it without military action.

That meant, McMaster said, working to enforce current UN sanctions and perhaps ratcheting them up. "And it also means being prepared for military operations if necessary," he said.

Trump said he believes China's President, Xi Jinping, has been putting pressure on North Korea over its missile and nuclear weapons programmes.

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The launch comes at a point of particularly high tension in the region.

Trump has sent a nuclear-powered submarine and an aircraft carrier to Korean waters.

President Donald Trump arrives to speak at the Pennsylvania Farm Show Complex and Expo Center in Harrisburg, Pa. Photo / AP
President Donald Trump arrives to speak at the Pennsylvania Farm Show Complex and Expo Center in Harrisburg, Pa. Photo / AP

The US and South Korea also started installing a missile defence system that is supposed to be partially operational within days.

Residents in the village of Seongju, where the missile defence system is being installed, scuffled with police on Sunday. About 300 protesters faced off against 800 police and succeeded in blocking two US Army oil trucks from entering the site, local media reported. A few residents were injured or fainted from the scuffle and were taken to a hospital.

The Terminal High-Altitude Area Defence system, or Thaad, is controversial in South Korea, and Presidential frontrunner Moon Jae In has vowed to reconsider the deployment if he wins the May 9 election.

He has said that the security benefits of THAAD would be offset by worsened relations with China, which is the country's biggest trading partner and is opposed to its deployment.

Trump raised eyebrows in South Korea last week when he said would make Seoul pay US$1 billion ($1.46b) for the missile defence system.

However, South Korean officials responded that the cost was for Washington to bear, under the bilateral agreement.

"National security adviser HR McMaster explained that the recent statements by President Trump were made in a general context, in line with the US public expectations on defence cost burden-sharing with allies," South Korea's Blue House said in a statement.

- Reuters