Tony Blair has urged western governments to recognise that they need to take an active role in the Middle East, saying the west should consider military options short of sending ground troops.
The former prime minister said there was a huge range of options available, including air strikes and drones as used in Libya.
Tony Blair also hit out at critics who linked the 2003 invasion of Iraq with the current violence in the country, blaming instead the West's failure to act in Syria.
The former prime minister, who led Britain into the US-led war to remove Saddam Hussein and is now a diplomatic envoy in the Middle East, also criticised the sectarianism of the government in Baghdad.
In a long article published on his website, Blair said arguments that there would be no crisis in the region if the Iraqi dictator had remained in power were "bizarre".
But he said the Syrian conflict had provided the Islamic militants seizing swathes of territory in northern Iraq with battle experience and a base from which to launch their attacks across the border.
Blair said action in Syria needed not be another invasion, but said moderate rebels fighting President Bashar al-Assad should be "given the support they need".
Pointing out that as many people had been killed in Syria as in the whole of Iraq since 2003, Blair said he would have supported military intervention in Syria some time ago, and suggested that there may have to be accommodation with President Assad.
Blair was prime minister between 1997 and 2007 and is now representative for the Middle East Quartet, comprised of the United Nations, the European Union, the United States and Russia.