President Donald Trump. Thomson Reuters President Donald Trump said on Thursday that his previous promise to respond to threats from North Korea with "fire and fury like the world has never seen" may not have been tough enough.

Trump added that North Korea should "get their act together" or it would be in trouble "like few nations have ever been."

"The people that were questioning that statement, was it too tough? Maybe it wasn't tough enough," Trump said outside his golf club in Bedminster, New Jersey.

"It's about time that somebody stuck up for the people of this country and for the people of other countries," Trump continued, later adding, "We're backed 100% by our military."

Trump's initial threat was met hours later by North Korea announcing it would consider a missile strike directed at Guam, a US territory in the Pacific home to 7,000 US military personnel and 160,000 civilians. North Korea also dismissed Trump's "fire and fury" comment as "nonsense."

"Well, I don't think they mean that," Trump said.

Acknowledging the unprecedented nature of his threats against North Korea, Trump said, "It's the first time they heard it like they heard it."

"If North Korea does anything in terms of even thinking about attack ... they can be very, very nervous," Trump said in response to a question about North Korea's threat. "Things will happen to them like they never thought possible."

Trump also attacked the Clinton and Obama administrations for failing to halt North Korea's nuclear program.

"The people of our country are safe," Trump said later. "Our allies are safe."

North Korean leader Kim Jong Un inspecting the long-range ballistic rocket Hwasong-12. Thomson Reuters

He added that North Korea's leader, Kim Jong Un, had "been pushing the world around for a long time," alluding to his aggression in the Western Pacific.

Defense Secretary Jim Mattis on Wednesday said North Korea "should cease any consideration of actions that would lead to the end of its regime and the destruction of its people."

North Korea has reasserted that it's considering shooting four missiles near Guam. Experts contacted by Business Insider said they found it unlikely that North Korea would launch such strikes — though they said the US had already suffered a blow to its credibility when Kim crossed Trump with his threat.