The Royal Australian Air Force (RAAF) combat mission over Syria has begun, Prime Minister Tony Abbott has confirmed.

The Federal Government on Wednesday announced Australia's air campaign against Islamic State (IS) would be expanded beyond Iraq to include parts of Syria.

"Our planes are now striking at terrorist targets inside Syria as well as in Iraq too," Mr Abbott said.

In a statement Defence said two RAAF F/A-18 Hornets, an air-to-air refuelling aircraft and an Early Warning and Control aircraft combined during the mission but no weapons were released.

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RAAF's Air Task Group commander Stu Bellingham said the aircraft searched points of interest for enemy activity in eastern Syria, reporting to the Combined Air Operations Centre.

"Daesh controls a large amount of territory in eastern Syria that serves as a source of recruitment and oil revenues, and as a base from which it continues to launch attacks into Iraq," he said, using another name for IS.

"The Hornets [used in the mission] were also prepared for any short notice high-priority tasking which could include surveillance and weapons release."

The Government has maintained the legal basis for the extended air strikes is the "collective self-defence" of Iraq.

"I emphasise that our aircraft will be targeting Daesh, not the Assad regime, evil though it is," Mr Abbott said on Wednesday.

"This is very much in Australia's national interest. Destroying this death cult is essential, not just to ending the humanitarian crisis in the Middle East but also to ending the threat to Australia and the wider world."

Syria has been embroiled in a vicious civil war since a popular uprising against dictator Bashar al-Assad in 2011.

More than 200,000 people have been killed and an estimated 4 million have been forced from their homes amid fighting between Mr Assad's forces, IS and other Islamist militants, and other rebel groups.

ABC/Reuters