Nigel Inkster cautioned the British government that getting embroiled in direct military action in Syria could antagonize President Bashar al-Assad’s allies, including Russia.
He also said the any military activity that takes place in Iraq is with the consent of the Iraqi government, which is not the case with Syria, and that any such activity could be deemed as “an act of war”.
"You can be confident that Assad's allies would be very quick to make this point. But from a military perspective the logic of such an engagement is inevitable because ultimately Syria is where this force needs to be defeated. The emphasis has to be on local actors, enabling local Syrian actors. They had some success previously [against ISIL] but then they had logistical problems, running out of equipment just at the point Isis was acquiring new supplies," he added.
The former intelligence director’s comments follow the US Senate approving President Barack Obama's plan to train and arm anti-government insurgents in Syria to fight ISIL terrorists.
The UK, who refers to targeting ISIL positions in Syria as complicated, has not yet launched airstrikes in Iraq or Syria.
The Takfiri ISIL militants have seized large swathes of Iraq and Syria. They have carried out heinous crimes in the two countries including mass execution of people.
Syria has been gripped by deadly violence since 2011. The Western powers and their regional allies -- especially Qatar, Saudi Arabia and Turkey -- are reportedly supporting the militants operating inside Syria.
More than 191,000 people have been killed in over three years of fighting in Syria, says the Office of the UN High Commissioner for Human Rights (OHCHR), calling the figure a probable “underestimate of the real total number of people killed.”