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 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 said.
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 said he hoped "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 said the situation had not dramatically changed over the last few days, and that Americans "should sleep well at night".
China has urged calm.
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.
While not confirmed, this was seen as one of the last obstacles to North Korea being a fully nuclear-armed state.
The tiny but important island of Guam
- The 541sq km volcanic and coral island is in the Pacific between the Philippines and Hawaii.
- It is a "non-incorporated" US territory, with a population of about 163,000.
- That means people born in Guam are US citizens, have an elected governor and House Representative, but cannot vote for a president in US national elections.
- US military bases cover about a quarter of the island. About 6000 personnel are based there and there are plans to move in thousands more.
- It was a key US base in World War II, and remains a vital staging post for US operations, giving access to potential flashpoints like the South China Sea, the Koreas and the Taiwan Straits.