President Trump on Thursday doubled down on his “fire and fury” warning to North Korea, suggesting any attack on Guam -- as threatened by Pyongyang -- would result in an unprecedented response from the U.S.

"Let’s see what [Kim Jong Un] does with Guam. He does something in Guam, it will be an event the likes of which nobody has seen before – what will happen in North Korea," Trump said.

At a separate appearance, Trump also pushed back against critics who suggested his comments earlier in the week were too forceful. Trump told reporters: “Maybe it wasn’t tough enough."

“They’ve been doing this to our country for a long time, for many years,” the president said of North Korea. “It’s about time that somebody stuck up for the people of this country and for the people of other countries. So if anything, that statement wasn’t tough enough.”

Trump made the comments at his New Jersey golf club, where he had a security briefing with Vice President Mike Pence, National Security Adviser H.R. McMaster and White House Chief of Staff John Kelly.

Asked by a reporter what’s tougher than threatening "fire and fury," Trump said: “You’ll see. You’ll see.”

“If North Korea does anything in terms of even thinking about attacking anybody that we love or we represent or our allies or us, they can be very, very nervous,” he said.

Trump wouldn’t say if he’s considering a preemptive strike on North Korea. “We don’t talk about that,” he said. “I never do.”

“What they’ve been doing, what they’ve been getting away with, is a tragedy and it can’t be allowed,” he said.

Tensions have escalated with North Korea over the last several days. On Tuesday, Trump reacted to reports that North Korea had produced a compact nuclear warhead by warning the country that it would face consequences if it threatened the United States.

“They will be met with fire and fury like the world has never seen,” Trump said.

The comment was questioned by some Republicans, including Arizona Sen. John McCain.

"I don't know what he's saying and I've long ago given up trying to interpret what he says,” McCain told KTAR radio. "It's not terrible but it's kind of the classic Trump in that he overstates things."

Meanwhile, congressional Democrats slammed Trump, calling his threat reckless.

“The president’s ‘fire and fury’ ad lib was not helpful,” said Rhode Island Democratic Sen. Jack Reed, the ranking member of the Senate Armed Services Committee. “Defusing the North Korea threat will take smart, steady leadership and stronger diplomatic ties with our key allies.”

House Democratic leader Nancy Pelosi called the comments “recklessly belligerent.”

Hours after Trump’s original warning, state media in North Korea reported that its leaders were seriously considering a plan to fire missiles at Guam. Pyongyang called Trump's threat a "load of nonsense."

But the White House has tried to present a unified front.

White House Press Secretary Sarah Huckabee Sanders said Wednesday that Kelly and members of the National Security Council were “aware of the tone of the statement of the president prior to delivery.”

On Wednesday, Defense Secretary Jim Mattis issued a threat to North Korea in a fiery statement of his own, warning the country not to "initiate" a conflict.

“The DPRK regime's actions will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates,” the defense secretary said.