Senate Armed Services Chairman John McCain said he took exception to Trump's comments, warning that the President might not be able to follow through with the threats he is making.
"I take exception to the President's comments because you've got to be sure that you can do what you say you're going to do," the Arizona Republican said in an interview with Phoenix radio station KTAR. "The great leaders I've seen don't threaten unless they're ready to act and I'm not sure President Trump is ready to act."
Trump issued his warning to North Korean leader Kim Jong Un on Tuesday during a photo op at the Trump National Golf Club in Bedminster, New Jersey.
"North Korea best not make any more threats to the United States. They will be met with fire and fury like the world has never seen... he has been very threatening beyond a normal state. They will be met with fire, fury and frankly power the likes of which this world has never seen before," he said.
McCain tried to be measured with his reaction, telling KTAR it was "classic Trump."
"It's not terrible what he said," McCain said. "It's kind of the classic Trump in that he overstates things."
But Democrats slammed Trump for a reaction they charged was "bombastic" and "unhinged."
"President Trump is not helping the situation with his bombastic comments," California Sen. Dianne Feinstein said in a statement.
New York Rep. Eliot Engel, the top Democrat on the House Foreign Affairs Committee, said Trump had undermined US credibility "by drawing an absurd red line."
"Make no mistake: North Korea is a real threat, but the President's unhinged reaction suggests he might consider using American nuclear weapons in response to a nasty comment from a North Korean despot," Engel said.
And Maryland Sen. Ben Cardin, the top Democrat on the Senate Foreign Relations Committee, compared Trump's comments to rhetoric that often comes out of Pyongyang.
"President Trump's comments were not helpful and once again show that he lacks the temperament and judgment to deal with the serious crisis the United States confronts," Cardin said. "We should not be engaging in the same kind of bluster and provocative statements as North Korea about nuclear war."
Democratic Sen. Chuck Schumer of New York similarly called out Trump's comments -- referring to them as "reckless."
"We need to be firm and deliberate with North Korea, but reckless rhetoric is not a strategy to keep America safe," Schumer said in a statement issued Tuesday.
Guam's Democratic Rep. Madeleine Bordallo told CNN Tuesday night that she takes the threats from North Korea "very seriously."
"I've always been very, very concerned about this," Bordallo told CNN's Anderson Cooper. She added that the US military has repeatedly assured her that Guam will be protected.
CNN and The Washington Post reported Tuesday that US intelligence analysts have assessed that North Korea has produced a miniaturized nuclear warhead, according to multiple sources familiar with the analysis of North Korea's missile and nuclear program.
Feinstein called on the Trump administration to engage North Korea in "high-level dialogue without any preconditions."
"Hopefully, Secretary (Rex) Tillerson is already discussing the possibility of reopening talks with our Asian partners during his current trip. In my view, diplomacy is the only sound path forward," she said.
Sen. Dan Sullivan, R-Alaska, told CNN's Erin Burnett that a preemptive strike against the North Korean regime would require Congress' approval.
"The administration has done a good job up until now working closely with the Congress on their broader strategy. But we're going to play an important role here," Sullivan said Tuesday night.
Sullivan noted that if North Korea were to attack the US, "the President, as commander in chief, has more authority to react to that."
Abraham Denmark, a former deputy assistant secretary of Defense for East Asia, said the mixed messages coming from the Trump administration were problematic because they could create confusion for both allies and adversaries.
"Our adversaries and our allies are getting very mixed messages from the Trump administration, and this is why you need to have experienced people in government," he said. "This is why diplomacy requires more than a Twitter account and some bravado -- you need to have real experienced diplomats coordinating all these messages."