Chinese and Australian warships had a standoff in the South China Sea

Three of Australia's warships were "challenged" by the Chinese navy in the South China Sea early April, 2018, according to an ABC report.

Defense sources told ABC that Australia's navy was en route to Vietnam when it encountered polite but "robust" challenges from the People's Liberation Army, but the specific nature of the challenges is not described. HMAS Toowoomba had departed from Kota Kinabalu, Malaysia, while HMAS Anzac and HMAS Success travelled through the South China Sea after leaving the Philippines.

It's believed the interaction happened around the same time China was conducting its largest-ever naval parade on April 12, 2018. The massive show of force involved 10,000 naval officers, 48 naval vessels, submarines, the country's only aircraft carrier.

During the event President Xi Jinping was on board one of the destroyers, overseeing the parade.

When questioned about the incident, Australia's Prime Minister Malcolm Turnbull wouldn't reveal any details.

"All I can say to you is we maintain and practice the right of freedom of navigation and overflight throughout the world," Turnbull said. "In this context, you're talking about naval vessels on the world's oceans including the South China Sea, as is our perfect right in accordance with international law."

The South China Sea is a highly contested and valuable region. It is a major shipping route and some claim it has more oil reserves thany any other area on the planet, except Saudi Arabia.

Numerous countries — including China, Brunei, Taiwan, Malaysia, Vietnam, and the Philippines — have territorial claims, making the South China Sea one of the most disputed places on the planet. For its part, China has been criticized for building artificial islands in the region and militarizing them with missile sites and air bases.

This isn't the only problem Turnbull has faced with China of late.

In 2017, Turnbull proposed a new law to target and broaden the definition of foreign interference, after a wave of claims regarding China's influence campaigns in Australia. The laws have been derided in China and since then the two countries have been sparring over strained diplomatic relations and China's growing influence in the Pacific.

This article originally appeared on Business Insider. Follow @BusinessInsider on Twitter.