US Secretary of State Rex Tillerson says there is no imminent threat from North Korea, despite Pyongyang saying it was considering strikes on the US Pacific territory of Guam.
Mr Tillerson also defended President Donald Trump, who has threatened North Korea with "fire and fury".
He said Mr Trump just wanted to send a clear message to North Korea.
Defence Secretary James Mattis reiterated that message by urging North Korea to halt its weapons programme.
He said Pyongyang "should cease any consideration of actions that would lead to the end of its regime and the destruction of its people".
"While our State Department is making every effort to resolve this global threat through diplomatic means, it must be noted that the combined allied militaries now possess the most precise, rehearsed and robust defensive and offensive capabilities on Earth," he said in a statement on Wednesday.
North Korea's actions "will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates", he added.
The president earlier on Wednesday boasted on Twitter about the US nuclear arsenal.
In early morning tweets posted from New Jersey, where he is on holiday, Mr Trump said 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 tweets came after escalating rhetoric between the two sides, with the island of Guam - home to US military bases, strategic bombers and about 163,000 people - the focus of attention.
Stopping in Guam for a refuelling stop after a trip to South East Asia, Mr Tillerson said he was hopeful a global "pressure campaign", involving both Russia and China, could lead to new dialogue with Pyongyang "about a different future".
He defended President Trump's earlier language, saying that North Korean leader Kim Jong-un did not understand diplomatic language, and a strong message was needed that he would understand.
The top US diplomat added the situation had not dramatically changed over the last few days, and that Americans "should sleep well at night".
China has urged calm.
As Donald Trump made his "fire and fury" threat against North Korea, the lone camera in the room zoomed in for a tight shot. It would have been telling to see the effect those words had on his advisers.
Did they nod knowingly or look surprised? Was this part of the plan or an off-the-cuff presidential remark they'd have to clean up?
A day later and thousands of miles away, Rex Tillerson had out his mop and broom. The secretary of state said Mr Trump was merely reasserting that the US would defend itself if attacked, using language North Korea would understand.
That's not exactly what the president said, however. He warned of retribution if Kim Jong-un continued to make threats - punishing words with action.
It could simply be unfortunate phrasing from a man who is sometimes loose with words. Mr Tillerson would not be the first presidential aide intent on framing Mr Trump's blunt statements in a softer light.
With the president touting US nuclear might and test-firing an ICBM, however, the secretary of state - already considered to have little sway in this administration - may find his efforts at clarification buried in a torrent of bluster.
North Korea has continued to pursue its nuclear and ballistic missile programmes despite successive rounds of UN sanctions.
On Tuesday, media reports in the US claimed the North had achieved its goal of making a nuclear warhead small enough to fit inside its missiles.
Source: BBC