The Australian military is carrying out "freedom of navigation" flights over disputed islands in the South China Sea, the BBC has uncovered.
The US insists on its right to make such flights and B-52 bombers flew over the area last month, angering China.
In a recent civilian flight, a BBC team intercepted radio communications showing the Australian military is also operating such flights in the area.
Australia's defence department confirmed the flights to the BBC.
In a statement, the Department of Defence said one of its P-3 Orion surveillance aircraft was carrying out what it called "a routine maritime patrol" as part of its efforts to maintain regional security and stability in the region.
"China navy China navy we are an Australian aircraft exercising international freedom of navigation rights, in international airspace in accordance with the international civil aviation convention, and the United Nations Convention on the Law of the Sea - over," said a message heard by the BBC's Rupert Wingfield-Hayes while on board a Philippines civilian aircraft.
China is locked in territorial disputes in the South China Sea. It claims large swathes of the South China Sea - an area defined by the "nine-dash line". Vietnam and the Philippines have both contested this claim.
China's foreign ministry spokesman Hong Lei said: "Freedom of navigation in the South China Sea is not a problem. Countries outside of this area should respect other countries' sovereignty and not deliberately make trouble."
The disputed Spratly Islands are regarded as one of the potential geopolitical flashpoints of the 21st Century, our correspondents say.
China has been using land reclamation to expand islands and is building runways on them, sparking outrage from its neighbours. The US has warned China to stop all land reclamation activity.
What is Freedom of Navigation?
The US Freedom of Navigation programme challenges what it deems to be "excessive claims" to the world's oceans and airspace.
It was developed to promote international adherence to the UN Convention on the Law of the Sea, even though the US has not formally ratified the treaty.
In 2013 and 2014, the US conducted Freedom of Navigation operations of different kinds against China, Malaysia, the Philippines, Taiwan, and Vietnam - each of whom occupies territory in the South China Sea.