A NATO member country has reportedly raised concerns about Donald Trump’s control of his country's nuclear weapons.

CNN reports that the country – a member of the nearly 70-year-old international military alliance – was concerned with the US President’s command of the nuclear launch system. The country's fears were somewhat alleviated after a briefing on the subject, a diplomatic source said.

Mr Trump has maintained a strained relationship with other NATO member states over the years, at one point calling the organisation “obsolete”. He has frequently demanded that partner countries "pay their fair share" for their mutually assured defence.

At a meeting in South Korea last month, however, Defence Secretary James Mattis vowed the US would protect its NATO allies – even in the event of a nuclear strike.

“Make no mistake – any attack on the United States, or our allies, will be defeated," he said. "And any use of nuclear weapons will be met with a massive military response that is both effective and overwhelming.”

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But even inside the US, legislators have raised concerns about Mr Trump’s control of the nuclear arsenal. At a hearing on Tuesday, Senators debated revoking the President's decades-old power to unilaterally mount a first strike.

"We are concerned that the President of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic that he might order a nuclear weapons strike that is wildly out of step with US national security interests," said Senator Chris Murphy.

Democrats and Republicans alike have raised questions about the President’s powers in light of his escalating threats to North Korea. In August, Mr Trump claimed he would meet North Korea with “fire and fury like the world has never seen”. In October, the threatened to “totally destroy” the country.

Chris Murphy we are concerned that Trump is 'so unstable and volatile'

This has left some Senators wondering if the US is on the brink of a nuclear conflict.

“Under existing laws, the president of the United States can start a nuclear war – without provocation, without consultation and without warning,” Senator Ed Markey said on Tuesday. “It boggles the mind.”

Mr Markey and California Representative Ted Lieu have already filed bills that would prohibit the President from launching a preemptive nuclear strike without agreement from Congress. Panel members at the hearing, however, warned against “legislative fixes” to the issue.

"The system is not a button that the President can accidentally lean on against the desk and immediately cause missiles to fly,” said Peter Feaver, a professor of public policy and political science at Duke University. He emphasised that the decision requires the support of everyone from the Secretary of Defence to the service members working in the missile silo.