China challenged Australian warships in South China Sea, reports say

As the three vessels traversed the hotly contested region on their way to Vietnam, they were confronted by the People's Liberation Army (PLA) navy, the Australian Broadcasting Corporation reported on Friday.

The ABC said that one Australian defense official, speaking on condition of anonymity, "insists the exchanges with the Chinese were polite, but 'robust'."

Prime Minister Malcolm Turnbull, in London for a meeting of the heads of Commonwealth nations, refused to confirm or deny the report.

"All I can say to you is that Australia asserts and practices its right to freedom of navigation throughout the world's ocean, including the South China Sea," he told reporters.

In a statement to CNN, the Australian Defense Department acknowledged the three vessels were in the South China Sea in recent weeks but wouldn't comment on "operational details" on the ships.

"The Australian Defense Force has maintained a robust program of international engagement with countries in and around the South China Sea for decades," the statement said. CNN has reached out to the Chinese Ministry of Foreign Affairs for comment.

The Australian ships are now conducting a three-day goodwill visit in Ho Chi Minh City in Vietnam.

While Australian air force jets have been challenged by the Chinese in the past, this was the first time, Carl Thayer told CNN, that he'd heard of any reports of navy vessels being confronted.

"That doesn't mean it hasn't occurred ... (But) the challenge is political, it's intimidatory and if you don't counter challenge then China can make the argument that the international community has acceded to China's claims," said Thayer, regional security analyst and emeritus professor at the University of New South Wales.

Australia, China relations in deep chill

The reported confrontation comes at a moment of frosty relations between Beijing and Canberra. Turnbull admitted to the diplomatic chill on April 12.

"There has been a degree of tension in the relationship which has arisen because of criticism in China of our foreign interference laws," he told local radio station 3AW at the time.

His remarks followed reports in local media that Australian ministers had been denied visas that would have allowed them to attend China's signature Boao Forum in Hainan province.

The Chinese government has objected strongly to a new set of laws being considered by Australia to tackle interference by foreign nations in their politics.

Although Turnbull stressed that those laws weren't targeted at any one country, the legislation came after a series of scandals over large donations to Australian politicians by Chinese businessmen.

"I would like to stress hereby again that we hope the Australian side will abandon the cold-war mentality and ideological bias, stop making irresponsible remarks and work with China," Chinese foreign ministry spokesman Geng Shuang said in February.

China tightens hold on volatile region

The South China Sea is one of the most hotly contested regions in the world. China claims a huge swathe of territory across the sea, overlapping the claims of Vietnam and the Philippines, among others.

Only last week the Chinese navy held its largest ever drills in the South China Sea, including a huge military parade overseen by Chinese President Xi Jinping.

China's only aircraft carrier the Liaoning took part in the display, launching J-15 fighter jets from the enormous ship's flight deck.

Speaking from the PLA destroyer Changsha, Xi called for further modernization of China's military to further his goal of creating a "world-class" force under the Communist Party's leadership.

To reinforce their claims in the region, China has constructed and militarized a series of artificial islands across the South China Sea, building airfields and radar stations.

The United States regularly conducts Freedom of Navigation Operations close to China's artificial bases. Last year, Arizona Senator John McCain suggested Australia's Navy take part alongside the US in those exercises.

"I would not try to tell the Australians what they need to do, but there are exercises where a number of nations join together --- we call it RIMPAC [Rim of the Pacific Exercise]--- that the Australians participate in. They're broad naval exercises," he said during a visit to Australia.