US President-elect Donald Trump said Thursday the country should "greatly strengthen and expand" its nuclear arsenal, a stark contrast to outgoing President Barack Obama's championing of a world without nuclear weapons.

"The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes," Trump said in a Twitter posting. It's unclear what made him take on the issue, but he met with a group of Pentagon officials a day earlier.

The remark suggests the incoming administration would provide active support for the Pentagon to modernize nuclear weapons, an expensive project that reportedly could cost as much as $1 trillion over the next 30 years.

Earlier Thursday, Russian President Vladimir Putin also called for bolstering "strategic nuclear forces," especially those capable of penetrating "existing and future missile defense systems." US media said the remarks by Trump and Putin suggest a new nuclear arms race.

Trump's remark represents a stark departure from Obama's policy on nuclear weapons.

Obama has championed a nuclear-free world, seeking to make the initiative a key legacy of his presidency. He launched the Nuclear Security Summit of world leaders aimed at reducing the stockpile of fissile material and keeping it out of the hands of terrorists, and the fourth and last Nuclear Security Summit was held in Washington earlier this year.

Obama even considered officially declaring the US won't use nuclear weapons first, even though he dropped the so-called nuclear "no first use" proposal in the face of concerns and objections from allies and senior officials.

On the contrary, Trump even suggested at one point during the campaign that South Korea and Japan could be allowed to develop their own nuclear weapons for self-defense so as to reduce American burden of helping defend the key Asian allies.

The remarks had been one of the biggest sources of criticism for Trump, fueling calls in South Korea for nuclear weapons development that have arisen in the wake of North Korea's fifth nuclear test in January.

Since the election, however, Trump has denied ever making those remarks. (Yonhap)