Mr Tillerson sought to downplay the escalating diplomatic crisis, saying Americans “should sleep well at night”.
“I do not believe that there is any imminent threat” from North Korea, he added.
His intervention came after Mr Trump told North Korea’s leadership it would be “met with fire and fury like the world has never seen” if it made more threats to the US.
Pyongyang in turn said it was “carefully examining the operational plan for making an enveloping fire at the areas around Guam”, an island in the Pacific home to around 6,000 US military personnel.
Mr Tillerson, speaking as he returned from a tour of Southeast Asia, said the President’s comments were an attempt to prevent “any miscalculation” by the North.
“What the President is doing is sending a strong message to North Korea in language that Kim Jong-un can understand, because he doesn’t seem to understand diplomatic language,” the Secretary of State said.
“I think the President just wanted to be clear to the North Korean regime on the US’ unquestionable ability to defend itself, will defend itself and its allies.”
But he also risked upping the ante as he warned that Guam was not the only place that was at risk from a missile strike.
The plane carrying Mr Tillerson from Malaysia to Washington included a scheduled stop in Guam to refuel.
He said re-routing his trip to avoid stopping in Guam had not been considered as “the North Korean missile capability can point in many directions, so Guam is not the only place that would be under threat”.
Mr Tillerson, who had been in Asia to work on a solution to the crisis, also insisted the Trump administration’s current strategy to deal with Mr Kim was working.
“The pressure is starting to show,” he said. “I think that’s why the rhetoric coming out of Pyongyang is beginning to become louder and more threatening.
“Whether we’ve got them backed into a corner or not is difficult to say, but diplomatically, you never like to have someone in a corner without a way for them to get out.”
Pressed on whether Pyongyang had a way out, with rhetoric from both sides suggesting they were on course for conflict, Mr Tillerson replied that the…