A DEAL to stop foreign fighters travelling through Turkey to side with Islamic State would be "nigh impossible" to enforce on the nation's porous borders, a defence expert says.
On Thursday, Prime Minister Tony Abbott signed a deal with Turkey to better enforce its borders to stem the flow of Australian foreign fighters through Turkey and into Iraq and Syria.
The arrangements would also extend to more intelligence-sharing on IS between the two governments, days after a similar deal was reached with Iran.
But Australian National University Strategic and Defence Studies Centre senior fellow Dr John Blaxland said it was unlikely the 900km Turkish border could be properly policed.
He said there was no reason to believe Turkey was not serious about the deal, but there was "no guarantee it would affect the flow (of fighters) dramatically".
However he said Turkish authorities could play a key role at sea and air ports, where official channels demanded travellers move through Customs and potential fighters could be stopped.
Dr Blaxland said while IS, or Daesh, shared the Sunni religion with most Turks, the Turkish and Australian governments had a common interest in stability in the Middle East.
Similarly, he said an intelligence sharing deal between Australia and Iran was a product of Iran's interest in stability, despite its human rights record.
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