The former head of the Australian Defence Force (ADF), Retired General Peter Gration, has signed an open letter to Prime Minister Tony Abbott opposing bombing raids in Syria.

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Australia is considering a request by the US to join its bombing campaign against the Islamic State (IS) group in Syria.

The open letter suggests bombing IS targets would potentially be illegal, could strengthen the organisation and divide the Australian community.

Retired General Gration, who was the Commander of the ADF from 1987 to 1993, told The World Today the move would be "inviting disaster".

"I believe this would be a strategically bad decision; in fact I would call this strategically dumb," he said.

"To commit us to what is complex and confused war with a centuries old religious conflict between the Sunnis and the [Shiites], the underlying issue, I think is really inviting disaster."

Retired General Gration said the US air strikes in Syria had not stopped IS and Australia's contribution would be "at best a marginal increase".

He said it would inevitably lead to a ground war.

The letter to the Prime Minister was signed by Retired General Gration and a number of doctors and academics.

The letter states: "The Government's objectives in bombing Syria should be made clear and evaluated against our moral obligation to preserve civilian life, together with the prospect that bombing will strengthen Islamic State, and will further divide the Australian community."

It calls on the Prime Minister to hold a debate in the Federal Parliament on the impact of a bombing campaign in Syria on Australia's national interests and on Syrian civilians.

It also asks the Government to seek legal opinion on the legality of a bombing campaign.

"This should make clear what Australia's likelihood of success before the International Court of Justice would be if another party raised arguments against the legality of the bombing," the letter said.

Retired General Gration said the humanitarian issue was significant.

"There are undoubtedly going to be more civilian casualties," he said.

"There'll be more refugees generated, there'll be more infrastructure damaged, and eventually getting Syria back on its feet will be quite difficult."

He said if Australia took part in bombing raids in Syria it would worse the humanitarian crisis currently playing out in Europe, as hundreds of thousands of asylum seekers seek a safe haven.

"If we step up increased airstrikes, it'll not only generate more casualties inside Syria, but will increase the flow of refugees from Syria outwards to Europe," he said.

Retired General Gration said he did not expect Mr Abbott would take heed of the letter, saying: "The Prime Minister's mind is already made up."