Issuing a new threat to North Korea, U.S. President Donald Trump demanded that North Korea "get their act together" or face extraordinary trouble. He said his previous warning of "fire and fury" if Pyongyang threatened the U.S. again might have been too soft.

"Maybe that statement wasn't tough enough," Trump said on Thursday.

Trump, speaking to reporters from the New Jersey golf resort where he's vacationing, said North Korea had been "getting away with a tragedy that can't be allowed." Still, he declined to say whether the U.S. was considering a pre-emptive military strike, arguing that his administration never discusses such deliberations publicly.

Trump's comments were his first since North Korea reacted to his "fire and fury" threat by announcing a detailed plan to launch a salvo of ballistic missiles to create an "enveloping fire" around the U.S. Pacific territory of Guam, a major military hub and home to U.S. bombers. Trump said it was time that 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.

Trump said the U.S. "of course" would always consider negotiations with North Korea, but added that negotiations with the North have failed for the last 25 years. He accused his four predecessors of failing to effectively address the North Korea problem.

Alluding to the threats against Guam, Trump said if North Korea took any steps to even think about an attack, it would have reason to be "very, very nervous."

"Things will happen to them like they never thought possible," Trump said. Of North Korean leader Kim Jong-un, Trump added: "He's been pushing the world around for a long time."

He also said that he expects China "will do a lot more" on North Korea but linked future trade policy on China to what Beijing does on North Korea. "If China helps us, I feel a lot differently toward trade," he said.