Canberra, Oct 17 (IANS): Australian jets have carried out at least two airstrikes against Islamic State (IS) militants, out of a total of 43 sorties by Royal Australian Air Force (RAAF), officials said Friday.

It is not known how many IS fighters died in the attacks, although defence officers said there had been "multiple" casualties, according to Xinhua.

Chief of Joint Operations Vice Admiral David Johnston told reporters here that the Australian forces had been forced to fly more missions over Iraq while the US planes had been diverted to Syria's Kobane town.

He revealed that in the past week, RAAF Super Hornets carried out the second airstrike and confirmed for the first time that the Australian strikes had killed IS fighters.

Australian planes has also been involved in strikes on an oil refinery, in a bid to destroy IS revenue sources, Johnston said.

Johnston said the impact of the strikes was direct, and a morale boost for the Kurdish fighters.

Johnston said he would not provide more information on the number of casualties due to potential exploitation by the "aggressive propaganda campaign" of IS.