A senior Syrian official has said President Bashar al-Assad threatened former Lebanese Prime Minister Rafik Hariri only months before his death.
Mr Khaddam once worked for Bashar al-Assad
Syria's former Vice-President, Abdul Halim Khaddam, said "Hariri received many threats".
The ex-Lebanese premier, who had become a critic of Syria, died in a bomb attack in Beirut in February.
A UN investigation has implicated Syria in the assassination. Syria denies it was involved.
UN investigator Detlev Mehlis said several sources had said they had been told by Mr Hariri that Mr Assad had threatened "to break Lebanon over [his] head", if he did not support the extension of Lebanese President Emil Lahoud's term.
Mr Khaddam told al-Arabiya television: "Assad told me he had delivered some very, very harsh words to Hariri... something like 'I will crush anyone who tries to disobey us'."
He said Syrian intelligence services could not have carried out such an assassination without the approval of Mr Assad, but he added: "We have to wait for the findings of the UN report."
Mr Khaddam, 73, who resigned as vice-president in June, told al-Arabiya television he was formally cutting links with the president, whom he accused of authoritarianism.
He told al-Arabiya he was "convinced that the process of development and reforms, be they political, economic or administrative, will not succeed".
Mr Khaddam used to be in charge of Syria's Lebanon policy. Damascus was the effective power in Lebanon for many years, until it withdrew its troops under international pressure this year.
The BBC's Kim Ghattas in Beirut says Mr Khaddam's intervention would be seen as very significant, important, and potentially dangerous, and would attract the interest of UN investigators.
She said it was unclear why Mr Assad's former colleague, who is now living in Paris, had chosen to speak out now, but that he may be positioning himself as an alternative to the Syrian president.