Air force war games off northern Australia involving 20 nations from Asia, Europe and the US are intended to send a message of deterrence amid deepening military co-operation between China and Russia, Defence Minister Richard Marles says.

Chinese and Russian bombers flying together last week were intercepted for the first time and warships from the two countries exercised together last month near the South China Sea, while Chinese troops also conducted combat drills in Belarus, a close Russian ally.