He also said that the entire region may "blow up" if a political solution is not found.
Brahimi, the former special representative on Syria for the United Nations and the Arab League, resigned in frustration last month after making little progress towards ending the brutal civil war, now in its fourth year.
When asked what he predicted would ultimately become of Syria, the veteran Algerian diplomat told der Spiegel: "It will become another Somalia. It will not be divided, as many have predicted. It's going to be a failed state, with warlords all over the place."
"Unless there is a real, sustained effort to work out a political solution, there is a serious risk that the entire region will blow up. The conflict is not going to stay inside Syria," he said.
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Brahimi, who previously served as a UN special representative for Iraq, Afghanistan and South Africa, oversaw two rounds of peace talks between President Bashar al-Assad's regime and the fractured political opposition-in-exile. But during two years in the post, he was unable to stop the bloodshed in a conflict that has killed more than 162,000 people and created millions of refugees.
He said the Islamic State of Iraq and the Levant (ISIL) -- a rebel group known for its ruthless tactics and suicide bombers -- "is active in both Syria and Iraq already, and Jordan is really struggling to continue resisting. Even Turkey. According to a senior Iraqi official, ISIS has carried out 100 operations in Syria and 1,000 operations in Iraq in just three months.
He added that there were 500 or 600 French nationals, and roughly the same number of Britons, fighting in Syria, together with several thousand non-Syrians.