WASHINGTON (REUTERS) - President Donald Trump said on Thursday (Feb 23) he wants to ensure the US nuclear arsenal is at the "top of the pack," saying the United States has fallen behind in its atomic weapons capacity.

In a Reuters interview, Trump also said China could solve the national security challenge posed by North Korea "very easily if they want to," ratcheting up pressure on Beijing to exert more influence to rein in Pyongyang's increasingly bellicose actions.

In his first comments about the US nuclear arsenal since taking office on Jan 20, Trump said he would like to see a world with no nuclear weapons but expressed concern that the United States has "fallen behind on nuclear weapon capacity."

"I am the first one that would like to see everybody - nobody have nukes, but we're never going to fall behind any country even if it's a friendly country, we're never going to fall behind on nuclear power.

"It would be wonderful, a dream would be that no country would have nukes, but if countries are going to have nukes, we're going to be at the top of the pack," Trump said.

Russia has 7,000 warheads and the United States, 6,800, according to the Ploughshares Fund, an anti-nuclear group.

"The history of the Cold War shows us that no one comes out 'on the top of the pack' of an arms race and nuclear brinkmanship," said Daryl Kimball, executive director of the independent Arms Control Association non-profit group.

"Russia and the United States have far more weapons than is necessary to deter nuclear attack by the other or by another nuclear-armed country," he said.

The new strategic arms limitation treaty, known as New Start, between the United States and Russia requires that by Feb 5, 2018, both countries must limit their arsenals of strategic nuclear weapons to equal levels for 10 years.

The treaty permits both countries to have no more than 800 deployed and non-deployed land-based intercontinental and submarine-launched ballistic missile launchers and heavy bombers equipped to carry nuclear weapons, and contains equal limits on other nuclear weapons.

Analysts have questioned whether Trump wants to abrogate New Start or would begin deploying other warheads.

In the interview, Trump called New Start "a one-sided deal."

"Just another bad deal that the country made, whether it's Start, whether it's the Iran deal... We're going to start making good deals," he said.

'WE'RE VERY ANGRY'

The United States is in the midst of a US$1 trillion (S$1.4 trillion), 30-year modernisation of its aging ballistic missile submarines, bombers and land-based missiles, a price tag that most experts say the country cannot afford.

Trump also complained that the Russian deployment of a ground-based cruise missile is in violation of a 1987 treaty that bans land-based American and Russian intermediate-range missiles.

"To me it's a big deal," Trump said.

Asked if he would raise the issue with Putin, Trump said he would do so "if and when we meet."

He said he had no meetings scheduled as of yet with Putin.

Speaking from behind his desk in the Oval Office, Trump declared that "we're very angry" at North Korea's ballistic missile tests and said accelerating a missile defense system for US allies Japan and South Korea was among many options available.

"There's talks of a lot more than that," Trump said, when asked about the missile defence system. "We'll see what happens. But it's a very dangerous situation, and China can end it very quickly in my opinion."

Trump's meeting with Japanese Prime Minister Shinzo Abe earlier this month in Florida was interrupted by a ballistic missile launch by North Korea.

Trump did not completely rule out possibly meeting Kim Jong Un at some point in the future under certain circumstances but suggested it might be too late. "It's very late. We're very angry at what he's done, and frankly this should have been taken care of during the Obama administration," he said.

According to Japanese news reports, the Japanese government plans to start debate over the deployment of a US missile defence system known as the Terminal High Altitude Area Defence, or Thaad, and the land-based Aegis Ashore missile defence system to improve its capability to counter North Korean ballistic missiles.

Japanese government sources told Reuters last year Japan may accelerate around US$1 billion of planned spending to upgrade its ballistic missile defences.

Trump vowed during his campaign to step up missile defence efforts and within minutes of his Jan 20 inauguration, the White House announced that the administration intended to develop a "state of the art" missile defence system to protect against attacks from Iran and North Korea.

It did not provide details on whether the system would differ from those already under development, its cost or how it would be paid for.