In an interview broadcast Friday by the al-Arabiyah television network, Abdel Halim Khaddam said Syria's president told Mr. Hariri he would destroy anyone who tried to hinder Syria's decisions.
Mr. Hariri was assassinated in February.
A United Nations probe of the assassination found evidence that Syrian agents were involved in the killing. Damascus has denied the charge.
Mr. Khaddam told al-Arabiya that no Syrian security service could have made the decision to kill Mr. Hariri without approval from above.
Some information for this report provided AFP, AP and Reuters.