The top US commander in the Pacific has told Congress the US may need to strengthen its missile defences, particularly in Hawaii, given the advancing threat from North Korea's missile and nuclear weapons programs.
Just before the entire US Senate receives a top-level White House briefing, Admiral Harry Harris said on Wednesday he believed Pyongyang's threats needed to be taken seriously.
Earlier, the US military moved parts of an anti-missile defence system to a deployment site in South Korea, triggering protests from villagers and by China - whose help is vital to agreeing and implementing tougher economic sanctions to try to persuade North Korea to abandon its weapons programs.
North Korea's growing nuclear and missile threat is perhaps the most serious security challenge confronting US President Donald Trump. He has vowed to prevent North Korea from hitting the US with a nuclear missile.
Harris told lawmakers the Terminal High Altitude Area Defense (THAAD) system would be operational "in coming days".
He said the defences of Hawaii were sufficient for now but could one day be overwhelmed, and suggested studying stationing new radar there as well as interceptors to knock out any incoming North Korean missiles.
"I don't share your confidence that North Korea is not going to attack either South Korea, or Japan, or the US ... once they have the capability," Harris said.
Washington has said all options are on the table, including military strikes, but officials have stressed that the current focus is on stepped-up sanctions on North Korea, which are expected to be discussed at a UN Security Council meeting on Friday chaired by US Secretary of State Rex Tillerson.
Harris conceded that North Korean retaliation to any US strikes could cause many casualties in South Korea, but added that there was the risk "of a lot more Koreans and Japanese and Americans dying if North Korea achieves its nuclear aims and does what (North Korean leader Kim Jong Un) has said it's going to do".
North Korea has vowed to strike the US and its Asian allies at the first sign of any attack on its territory.
In a show of force, the US is sending the USS Carl Vinson aircraft carrier group to waters off the Korean peninsula, where it will join the USS Michigan, a nuclear submarine that docked in South Korea on Tuesday. South Korea's navy has said it will hold drills with the US strike group.
The earlier-than-expected steps to deploy the missile defence system were denounced both by China, where the foreign ministry vowed Beijing would "resolutely take necessary steps to defend its interests".
But Harris, said he's encouraged by signals from China that it would help address North Korea's threatening behaviour, but cautions "it's early days".
"I'm encouraged. And I believe Kim Jong Un has noticed that there's a change afoot with regard to China, and I think that's important," Harris said.
North Korea's foreign ministry called US attempts to make Pyongyang give up its nuclear weapons through military threats and sanctions "a wild dream" and like "sweeping the sea with a broom".