The four Australians killed in Syria's civil war since August were fighting Assad forces and not providing humanitarian aid as claimed, jihadist websites say.

Posts on online forums, including some linked with the al-Qaeda affiliate Jabhat al-Nusra, were clear indications that the men had been involved in the civil conflict, a leading US academic said.

Circumstances surrounding death unclear: Sammy Salma.

Sydney teacher Sheikh Mustapha al Majzoub was killed in a rocket attack in August. Three other Australians - Roger Abbas, Yusuf Toprakkaya and Sammy Salma - have also been killed.

While Mr Toprakkaya is the only Australian confirmed as travelling to Syria to fight, the online forum posts claim all four had been fighting against the government of President Bashar al-Assad.