Veteran diplomat Lakhdar Brahimi, who stood down last week, also said that the conflict would “blow up” and spread beyond Syria’s borders.
His warning came as a Norwegian vessel with the first part of Syria’s chemical weapons arsenal left the country.
The operation to destroy Syria’s chemical weapons arsenal is several months behind schedule.
Mr Brahimi made the comments in an interview with German newspaper Der Spiegel in one of his first interviews since stepping down.
“It will be become another Somalia,” he warned. “It will not be divided, as many have predicted. It’s going to be a failed state, with warlords all over the place.”
He criticised Iran and Russia for supporting the Syrian government, and said President Bashar al-Assad was “100% aware” of the way the war was being conducted.
“He knows a hell of a lot. Maybe he doesn’t know every single detail of what is happening, but I’m sure he is aware that people are being tortured, that people are being killed, that bombs are being thrown, that cities are being destroyed,” he said.
More than 162,000 people have been killed since the Syrian conflict started in 2011, with millions forced to flee their homes.
‘Worse than Afghanistan’
Mr Brahimi said that both the Syrian government and the opposition had committed war crimes “every day” and warned that “people will be held responsible one day”.
The Algerian diplomat said the conflict would destabilise countries close to Syria and further afield.
“The UN Security Council had no interest in Afghanistan, a small country, poor, far away. I said one day it’s going to blow up in your faces. It did. Syria is so much worse!” he said.
Mr Brahimi chaired two rounds of peace talks between the Syrian government and the opposition movement in Geneva, but the negotiations ended in deadlock, prompting his resignation.
Weapons deadline looms
On Sunday, a Norwegian cargo ship carrying about 500 tonnes of chemical weapons from the Syrian government’s arsenal set sail for Finland and the United States.
The operation to completely destroy Syria’s chemical weapons stockpile was meant to have been completed by 30 June.
However, the Organisation for the Prohibition of Chemical Weapons (OPCW) says this deadline will now not be met.
Correspondents say fighting and the threat of attack by rebel groups has severely delayed the removal of the weapons.