US Indo-Pacific commander Admiral John Aquilino recently flew over the South China Sea and says China has built and militarised several artificial islands.
"They are full-fledged offensive bases," he told reporters in Darwin on Wednesday.
"Runways, hangers, barracks, anti-aircraft missiles, anti-ship missiles, jamming capability, hangers for fighter aircraft, bomber aircraft ... That is fact."
Asked about the military threat to Australia's north, including Darwin, Admiral Aquilino said, "There is certainly a threat".
"Today in the Indo-Pacific, it hosts the most concerning security challenge that we have faced in a number of years."
The Admiral said he would be in Australia for several days to work closely with the Chief of the Australian Defence Force General Angus Campbell on all aspects of the two nations' military collaboration.
"It should be concerning to nations that are interested in changing and potentially destabilising the region," he said.
The Admiral leads the largest US military command with 380,000 personnel across 36 nations.
Reflecting on Australia's recently announced space command, he said the United States would be working through AUKUS with Australia and the United Kingdom to strengthen defence capabilities in the space and cyber domains.
"Some of the US team is coming to Australia (tomorrow) to synchronise our exercise, operations and effects," he said.
"Critically important. We have no separation of domain between Australia and the US."
The Admiral said the US would also increase its military collaboration with Australia and other nations to maintain stability in the region.
"We want to be able to join together quickly and operate immediately," he said while inspecting a US Marine contingent based in Darwin.
The seasonal force, which also includes the US Army for the first time, is expected to grow to 2200 servicemen and women over the next few months.
AUKUS is a trilateral security pact between Australia, the US and the UK in the Indo-Pacific region.
The US Indo-Pacific Command is also the nation's oldest combatant command.
Admiral Aquilino oversees all US military activities, including the army, marines, airforce and navy, in the Indo-Pacific, covering 14 time zones and more than 50 per cent of the world's population.
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