Fifty-five people were killed and 372 were wounded when two car bombers blew themselves up in the capital Damascus on 10 May.
It was the deadliest attack on the city since the uprising against President Bashar al-Assad started last year.
Mr Ban also said the death toll from the crisis now stands at 10,000 people.
"Very alarmingly and surprisingly, a few days ago, there was a huge serious massive terrorist attack. I believe that there must be Al-Qaeda behind it. This has created again very serious problems," Mr Ban said on Thursday.