Tony Blair hit out on Sunday 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".
He condemned the sectarianism of Prime Minister Nuri al-Maliki's government, who he said had "snuffed out what was a genuine opportunity to build a cohesive Iraq".
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".
Meanwhile extremist groups, whether in Syria or Iraq, "should be targeted, in coordination and with the agreement of the Arab countries", he said, saying the United States should be considering military strikes in Iraq.
"By all means argue about the wisdom of earlier decisions. But it is the decisions now that will matter," he wrote.
"The choices are all pretty ugly, it is true. But for three years we have watched Syria descend into the abyss and as it is going down, it is slowly but surely wrapping its cords around us pulling us down with it.
"We have to put aside the differences of the past and act now to save the future. Where the extremists are fighting, they have to be countered hard, with force."
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.
