|Trump promises 'trouble' if N Korea attacks|
Mr Trump also referenced his recent "fire and fury" warning to 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, maybe that statement wasn't tough enough."
The President was speaking ahead of a meeting with security advisers.
"If North Korea does anything in terms of even thinking about an attack on anybody that we love or represent, or our allies or us, they should be very, very nervous."
:: Ramping up rhetoric risks miscalculation between US and North Korea
President Trump said Kim Jong Un has been "pushing the world around for a long time," and warned that things will "happen to them like they never thought possible".
He also added that he thinks China can do more to pressure Pyongyang to end its nuclear weapons programme.
He said: "I have great respect for China and Russia and what they did on sanctions... I think China can do a lot more, yes, and I think China will do a lot more."
He sent a reassuring message to the US, saying: "The people of our country are safe, our allies are safe, and I will tell you this: North Korea better get their act together, or they're gonna be in trouble like few nations have ever been in trouble in this world."
The President's comments are the latest escalation in an ongoing spat between the two nations.
Mr Trump warned North Korea on Tuesday that it would be met with "fire and fury" if it issued anymore threats.
:: Why is North Korea threatening to strike Guam?
Pyongyang responded by saying the President's warning was a "load of nonsense", and said on Wednesday that it would have a fully developed plan for striking Guam with intermediate range missiles by mid-August.