The leaders of both countries on Thursday warned of retaliatory actions if provoked by an attack as tensions escalate day by day.
North Korea said it planned to launch four missiles to strike the US territory of Guam, which was the launch site for recent US military flights over North Korea.
Responding to the threat, Mr Trump said North Korea should be "very, very nervous" if it even thinks about attacking the USA.
"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 going to be in trouble like few nations ever have been in trouble in this world," he said.
"Maybe it wasn't tough enough. They've been doing this to our country for a long time, for many years. And it's about time that somebody stuck up for the people of this country and for the people of other countries," he added.