North Korea has said it is considering carrying out missile strikes on the US Pacific territory of Guam.
The North's official news agency said on Tuesday the plan involved firing medium-to-long-range rockets at Guam, where US strategic bombers are based.
US Secretary of State Rex Tillerson, flying into Guam, said there was no imminent threat from North Korea.
He also defended President Donald Trump, who on Tuesday threatened Pyongyang with "fire and fury".
Mr Tillerson said that North Korean leader Kim Jong-un did not understand diplomatic language, and a strong message was needed that he would understand.
"I think the president just wanted to be clear to the North Korean regime that the US… will defend itself and its allies," he said.
On Wednesday, Mr Trump tweeted that the US nuclear arsenal was "more powerful than ever before", but added he was hopeful "we will never have to use this power".
The recent exchanges mark a sharp rise in rhetoric between Washington and Pyongyang, but China has urged calm.
A foreign ministry statement quoted by Reuters called on all sides to avoid words or actions which might escalate the situation and to make greater efforts to resolve the issue via talks.
The UN recently approved further economic sanctions on North Korea, which Pyongyang said were a "violent violation of our sovereignty", warning the US would "pay a price".
On Wednesday, the official KCNA news agency said North Korea was "carefully examining the operational plan for making an enveloping fire at the areas around Guam" using its domestically made medium-to-long-range Hwasong-12 missiles.