Australian air force patrols flying over the South China Sea are now being routinely challenged by the Chinese military in a sign of the growing stranglehold Beijing has over the strategically vital waters.

The Chief of the Air Force, Air Marshal Leo Davies, said on Wednesday that Australian surveillance patrol flights over the regional flashpoint had increased "slightly" and the RAAF would continue doing so as was its right under international law.

A People's Liberation Army J-11 fighter on exercises over the South China Sea. Credit:China Military Online

In a wide-ranging interview, Air Marshal Davies also said the RAAF would be stretched if asked to carry out sustained bombing raids in Libya on top of the campaign in Iraq and Syria against the Islamic State terror group.

Those remarks came after Washington flagged the need to tackle the Islamic State's growing presence in Libya and Foreign Minister Julie Bishop said she would "listen" to discussions about a possible greater coalition effort in the oil-rich, north African country. But Ms Bishop reiterated that Australia was already making a significant contribution to the fight against Islamic State and it was up to other countries to do more.