US Defence Secretary James Mattis has said America still hopes to solve the North Korea crisis using diplomacy.
After days of fiery rhetoric from both the US and North Korea, Mr Mattis said war would be "catastrophic" and that diplomacy was gaining results.
Pyongyang on Thursday announced it was finalising a plan to fire four missiles near the US territory of Guam.
Earlier, President Donald Trump said North Korea should be "very, very nervous" if it acted against the US.
He said the regime would be in trouble "like few nations have ever been" if they do not "get their act together".
Tensions have escalated rapidly in recent weeks after North Korea tested two intercontinental ballistic missiles in July.
It was further angered by a subsequent UN decision to increase economic sanctions against it.