"Maybe that statement wasn't tough enough," Trump said in the latest round of an escalating exchange of threats between the two nuclear-armed nations.
Speaking to reporters in New Jersey, Trump said North Korea has been "getting away with a tragedy that can't be allowed." Still, he declined to say whether the U.S. is considering a pre-emptive military strike, arguing that his administration never discusses such deliberations publicly.
Trump spoke after North Korea intensified its own rhetoric by announcing a detailed plan to launch a salvo of ballistic missiles toward the U.S. Pacific territory of Guam, a major military hub and home to U.S. bombers. That announcement had been a response to Trump's threat that the North would face "fire and fury like the world has never seen" if it threatened the U.S. again.
RELATED: North Korea details plan to fire missiles toward Guam
On Thursday, Trump said it is time somebody stood up to the pariah nation.
"North Korea better get their act together or they are going to be in trouble like few nations have ever been in trouble," Trump said, flanked by Vice President Mike Pence. "It may very well be tougher than I said."
Trump spoke after meeting with national security advisers at the golf resort where he's vacationing. He said the U.S. "of course" would always consider negotiations with North Korea, but added that talks with the North have failed for the last 25 years. He said that China, the North's biggest trading partner, needs to do more to apply pressure - and predicted that it will.
The threatened attack near Guam, if carried out, would be the North's most provocative missile launch to date. The North said it is finalizing a plan to fire four of its Hwasong-12 missiles over Japan and into waters around the tiny island, which hosts 7,000 U.S. military personnel on two main bases and has a population of 160,000.
Japan and South Korea vowed a strong reaction if the North were to go through with the plan. Trump added his voice on Thursday, insisting that if North Korea took any steps to even think about an attack, it would have reason to be nervous.
"Things will happen to them like they never thought possible, OK?" Trump said. Of North Korean leader Kim Jong Un, Trump added: "He's been pushing the world around for a long time."
North Korea said the plan, which involves the missiles hitting waters 30 to 40 kilometers (19 to 25 miles) from the island, could be sent to leader Kim for approval within a week or so. It would be up to Kim whether the move is actually carried out. But the extreme specificity of the plan suggested it was designed to show North Korea is actually plotting a launch.
The report said the Hwasong-12 rockets would fly over Shimane, Hiroshima and Koichi prefectures in Japan and travel "1,065 seconds before hitting the waters 30 to 40 kilometers away from Guam." It said the Korean People's Army Strategic Force will finalize the plan by mid-August, present it to Kim Jong Un and "wait for his order."
"We keep closely watching the speech and behavior of the U.S.," it said.
It is unclear whether - or exactly why - North Korea would risk firing missiles so close to U.S. territory. Such a launch would almost compel the United States to attempt an intercept and possibly generate further escalation.
North Korea, no stranger to bluffing, frequently uses extremely bellicose rhetoric with warnings of military action to keep its adversaries on their heels. It generally couches its threats with language stating it will not attack the United States unless it has been attacked first or has determined an attack is imminent.
But the statement raised worries amid threats from both sides.
South Korea's military responded by saying North Korea will face a "stern and strong" response from Washington and Seoul. Taking it a step further, Japan's Defense Minister Itsunori Onodera told parliament a missile attack on the U.S. territory would be a Japanese national emergency because it would threaten Japan's existence as a nation.
Guam lies about 2,100 miles (3,400 kilometers) from the Korean Peninsula, and it's extremely unlikely Kim's government would risk annihilation with a pre-emptive attack on U.S. citizens. It's also unclear how reliable North Korea's missiles would be against such a distant target, but no one was writing off the danger completely.
Washington has been testing its missile defenses in response to the North's stepped-up development and the current escalation of tensions could lead to pressure for the U.S. military to try to shoot down the North's missiles in midflight if they are heading toward Guam.
That would likely open up a set of very major problems, including the possibility of both a very high-profile failure or a miscalculation of Washington's intentions and a more deadly pre-emptive strike by the North - which has missiles able to hit Tokyo and conventional weapons that could devastate South Korea's capital of Seoul.
The Hwasong-12, which was revealed for the first time at a military parade in April, is an intermediate-range ballistic missile that is believed to have a radius of more than 3,700 kilometers (2,300 miles). It can be fired from mobile launchers, making it hard to detect and destroy on the ground.
By launching a salvo of four, the North would be attempting to make it harder for the U.S. to intercept all of the incoming missiles. Its stated flight path over Japan is also very aggressive - it has recently tried to avoid flying over neighboring countries by shooting its missiles up at a very high angle to land in the ocean.