U.S. President Donald Trump doubled down on his warnings to North Korea on Thursday, saying his threat to rain "fire and fury" on the isolated country maybe "wasn't tough enough."

Responding to North Korea's dismissal of his dire warnings as "nonsense," Trump told reporters, according to AFP, "Maybe it wasn't tough enough."

"They've been doing this to our country for a long time, many years," the President added. "It's about time that somebody stuck up for the people of this country and for the people of other countries. So if anything, maybe that statement wasn't tough enough."

Trump warned North Korea earlier this week it faced "fire and fury like the world has never seen" if it continued to threaten the United States with its missile and nuclear programs.

North Korea was unfazed by Trump’s warning, and responded by threatening a missile attack on Guam, a tiny U.S. territory in the Pacific.

On Wednesday, Pyongyang reiterated its threat and even said the plans to attack Guam would be ready by mid-August

Trump on Thursday told reporters "there are no mixed messages."

He said North Korea "should be very, very nervous. Because things will happen to them like they never thought possible, okay."

He said the United States remained open to negotiations and once again suggested that he expected China to do more to bring North Korea into line.

"I think China can do a lot more. I think China will do a lot more," added Trump, then said, "I will tell you this, North Korea better get their act together or they're going to be in trouble like few nations have ever been in trouble in this world, okay?"

The President’s remarks earlier this week followed reports that North Korea has produced a nuclear warhead small enough to fit onto an intercontinental ballistic missile (ICBM), which could strike parts of the U.S.

Late last week, the UN Security Council voted to place new sanctions on North Korea in response to its recent missile tests.