Russia could be more willing to deploy nuclear weapons today than the Soviet Union ever was during the Cold War, US Defense Secretary Ashton Carter warned Monday.

Speaking at Minot Air Force Base in North Dakota near the Canadian border, he accused Moscow of "nuclear saber-rattling," expressing concerns over Russia's push to overhaul its atomic weapons systems.

It "raises serious questions about its leaders' commitment to strategic stability, their regard for long-established accords against using nuclear weapons, and whether they respect the profound caution that Cold War-era leaders showed with respect to brandishing nuclear weapons," Carter told troops.

He was referring to Russian nuclear exercises and President Vladimir Putin's more strident nuclear rhetoric in recent months.

The Pentagon chief also described North Korea as an emerging nuclear threat.

But he praised China, whose military activities he has frequently criticized, for its nuclear conduct.

China "conducts itself professionally in the nuclear arena despite growing its arsenal in both quality and quantity."

Carter is traveling across the United States this week, highlighting America's nuclear capabilities as well as some of its ailing infrastructure, such as missile silos built in the 1950s.

The Pentagon is pushing ahead with plans to replace or modernize all three legs of its nuclear "triad" -- intercontinental ballistic missiles (ICBMs), submarines and bombers -- at a cost experts estimate will hit $1 trillion over the next 30 years.

The future of America's nuclear force is in the spotlight now more than it has been for years, thanks to Russian aggression along the border with Ukraine, North Korea's push to build a nuclear missile, and remarks by Republican presidential candidate Donald Trump suggesting overhauls to longstanding US nuclear policy.

- Minuteman missiles -

America's massive nuclear reinvestment comes despite President Barack Obama's memorable speech in Prague in 2009, when he called for the elimination of nuclear weapons, a call that helped him win the Nobel Peace Prize.

But he also said the United States would maintain a "safe, secure and effective" nuclear arsenal as long as the weapons exist.

"We're now beginning the process of correcting decades of under-investment in nuclear deterrence," Carter said, speaking in an aircraft hangar in front of a B-52 superbomber with six cruise missiles strapped beneath each of its vast wings.

Minot Air Force Base is one of three facilities across windswept rural America that oversee the US fleet of more than 400 Minuteman III ICBMs.

The weapons were first designed in the 1960s and the United States plans to replace them all over the coming two decades with a new missile system so far called the Ground Based Strategic Deterrent.

For Minot, that means its Minuteman missiles will be switched out and numbers reduced slightly under a 2010 deal with Russia.

Minot is also home to several hundred air-launched cruise missiles, each carrying a warhead packing more explosive yield than 10 of the type of bomb dropped on Hiroshima at the end of World War II.