By Aaron Bunch In Darwin For Australian Associated Press

Published: 03:57 EDT, 23 March 2022 | Updated: 17:23 EDT, 23 March 2022

Australia's vast north is open to attack amid the most concerning security challenge the United States' top military commander in the region has seen in recent years.

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.'

The new intelligence comes as a B-2 Stealth Bomber was pictured landing at the Amberley Air Force Base, 50km southwest of Brisbane in Queensland.  

Admiral John Aquilino will spend several days working closely with the Chief of the Australian Defence Force General on all aspects of the two nations' military collaboration (pictured, US Marines at the ADF Robertson Barracks in Darwin on Wednesday)

Asked about the military threat to Australia's north, including Darwin, Admiral Aquilino said, 'There is certainly a threat' (pictured, US Marines in Darwin on Wednesday)

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.

US Indo-Pacific commander Admiral John Aquilino recently flew over the South China Sea and says China has built and militarised several artificial islands (pictured)

A B-2 Stealth Bomber was pictured landing at the Amberley Air Force Base, 50km southwest of Brisbane in Queensland on Wednesday

US Indo-Pacific commander Admiral John Aquilino (pictured in Darwin on Wednesday) said China has built and militarised several artificial islands in the South China Sea

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.

The Admiral said the US would also increase its military collaboration with Australia and other nations to maintain stability in the region (pictured, US Marines in Darwin on Wednesday)

In September, Defence Minister Peter Dutton (pictured) warned that the risk of fighting with China 'shouldn't be discounted' and was 'up to the Chinese'

It comes as thousands of American soldiers trained to use surface-to-air missile systems, High Mobility Artillery Rockets and unmanned aircraft are bound for Australia as tensions grow with China.

The rotational US Marine force of about 2,200 servicemen and women will be based in the Northern Territory during the upcoming dry season from September.

It is the 11th deployment to the Top End and for the first time will include 250 US Army personnel, the Australian Defence Force announced.

Defence Minister Peter Dutton in September warned conflict with China 'shouldn't be discounted' and last week told the US Studies Centre Beijing may look to annex Taiwan while the world is preoccupied with Russia's invasion of Ukraine. 

Marine Rotational Force-Darwin is a US Force Posture Initiative designed to deepen inter-operability between the Australian Defence Force and US military. Members of the seventh rotation deployed in 2018 are pictured

The marine deployment will complement that Enhanced Air Co-operation program between the Royal Australian Air Force and United States Air Force. 

About 1000 Marines have already landed in Darwin.

They will train with the ADF to ensure they are able to respond to crises in the region.

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. 

About 1000 Marines have already landed in Darwin. They will train with the ADF to ensure they are able to respond to crises in the region (pictured is soldiers of the US Army in Lithuania)

Nuclear submarines are powered by nuclear reactors which produce heat that creates high-pressured steam to spin turbines and power the boat's propeller. They can run for about 20 years before needing to refuel, meaning food supplies are the only limit on time at sea.

The boats are also very quiet, making it harder for enemies to detect them and can travel at top speed - about 40kmh - for longer than diesel-powered subs.

The first nuclear submarines were put to sea by the United States in the 1950s. They are now also in use by Russia, France, the United Kingdom, China, and India. A senior US defence official told reporters in Washington DC: 'This will give Australia the capability for their submarines to basically deploy for a longer period, they're quieter, they're much more capable. 'They will allow us to sustain and to improve deterrence across the Indo-Pacific.'

Zack Cooper, a senior fellow with the American Enterprise Institute, said nuclear submarines would hugely boost Australia's military capability. They are going to be much, much more capable in the large, expansive ocean that is Australia has to deal with,' he told the ABC.

Will Australia have nuclear weapons?  

Scott Morrison made it clear that the nuclear-power submarines will not have nuclear missiles on board. Australia has never produced nuclear weapons and signed the Treaty on the Non-Proliferation of Nuclear Weapons in 1973 which prevents non-nuclear states which don't already have them from developing nuclear weapons.

Mr Morrison also said the Australia has no plans to build nuclear power stations which are widely used around the world. 'But let me be clear, Australia is not seeking to acquire nuclear weapons or establish a civil nuclear capability,' he said. 'And we will continue to meet all our nuclear non-proliferation obligations.'

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