Foreign Minister Julie Bishop has warned Beijing against militarising the South China Sea following reports that China has installed missile systems in the Spratly Islands for the first time.

Key points: China reportedly installed anti-ship cruise missiles and surface-to-air missiles in South China Sea

China reportedly installed anti-ship cruise missiles and surface-to-air missiles in South China Sea The White House said it had raised its concerns directly with Beijing

The White House said it had raised its concerns directly with Beijing Multiple countries have competing claims in the South China Sea

US news network CNBC reported on Thursday that China had installed anti-ship cruise missiles and surface-to-air missile systems on three outposts in the South China Sea, citing sources with direct knowledge of US intelligence.

Ms Bishop would not say if the Australian Government had intelligence confirming that — but said if the reports were accurate, then the Government would be worried.

"If the media reports are accurate then the Australian Government would be concerned because this would be contrary to China's stated aspiration that it would not militarise these features," Ms Bishop said.

"China of course has a unique responsibility as a permanent member of the Security Council to uphold peace and security around the world.

"And any action to militarise features in the South China Sea would go against that responsibility and that role."

The United States has also warned China that militarising the sea will have consequences.

Asked about the report, White House spokeswoman Sarah Huckabee Sanders told a regular news briefing: "We're well aware of China's militarisation of the South China Sea."

"We've raised concerns directly with the Chinese about this and there will be near-term and long-term consequences."

Ms Huckabee Sanders did not say what the consequences might be.

A US official, speaking on condition of anonymity, said US intelligence had seen some signs that China had moved some weapons systems to the Spratly Islands in the past month or so, but offered no details.

CNBC quoted unnamed sources as saying that according to US intelligence assessments, the missiles were moved to Fiery Cross Reef, Subi Reef and Mischief Reef in the Spratly Islands within the past 30 days.

They would be the first Chinese missile deployments in the Spratlys, where several Asian countries including Vietnam and Taiwan have rival claims.

The deployed surface-to-air missiles could target aircraft, drones and cruise missiles. ( China Ministry of National Defence )

China's foreign ministry said it has irrefutable sovereignty over the Spratlys and that necessary defensive deployments were for national security needs and not aimed at any country.

"Those who do not intend to be aggressive have no need to be worried or scared," Chinese foreign ministry spokeswoman Hua Chunying said.

China's defence ministry did not respond to a request for comment.

CNBC said the YJ-12B anti-ship cruise missiles allowed China to strike vessels within 295 nautical miles.

It said the HQ-9B long-range, surface-to-air missiles could target aircraft, drones and cruise missiles within 160 nautical miles.

Subi Reef, one of China's artificial islands in the South China Sea. ( CSIS AMTI: Reuters )

Eric Sayers, a former consultant to the commander of the US Pacific Command, called the missile deployment "a major escalation".

"When China sees that it can get away with these types of actions with little cost — as they did all through 2015 and 2016 — it only makes it more likely they will keep pressing," Mr Sayers said.

"China sees its participation in the exercise as a sign of its acceptance among the world's maritime powers but Beijing should not be allowed to militarise this open maritime domain and still be honoured as a welcomed member of the maritime community."

Last month, US Admiral Philip Davidson, nominated to head US Pacific Command, said China could use its "forward operating bases" in the South China Sea to challenge the US regional presence and "would easily overwhelm the military forces of any other South China Sea claimants".

Reuters