But the president on Wednesday vowed severe punishment if any national was found to have been involved.
In a rare interview with CNN, al-Assad called on America to stop blaming Syria for being unable to lock down its borders to fighters entering Iraq.
He said the real problem was the "chaos" resulting from the resistance to the US-led occupation of that country.
Al-Assad, who spoke in English and Arabic in his first major interview in three years, said he did not order al-Hariri's killing or believe such a crime could have been carried out by a Syrian without his knowledge.
"We are more confident ... that Syria has nothing to do with this crime," al-Assad said days before the release of a UN report on a investigation into al-Hariri's 14 February assassination.
"So far there is no material evidence of Syrian involvement," al-Assad said in the interview conducted hours before Syrian authorities announced the suicide of Interior Minister Ghazi Kanaan.
Kanaan's death, which Syrian opponents suggested could be murder to cover up high-level involvement in the assassination, shocked Syrians, and the government felt compelled to stress that his death would not affect the country's political stability.
Asked whether he would hand over any Syrians implicated in the killing to an international tribunal, Assad said: "Yes. If implicated, they should be punished. International, or Syrian, whatever. If they're not punished internationally, they will be punished in Syria.
Al-Assad says Syria had nothing
to do with Rafiq al-Hariri's death
"If indeed there is a Syrian national implicated in it, he would be considered as a traitor and most severely punished," he added.
Syria had nothing to gain from killing al-Hariri, Assad said, and would instead put itself under great pressure if it did order his death.
"This is against our principles and my principles and I would never do such a thing in my life," al-Assad said. "What do we achieve? I think what happened targeted Syria ... I would never do it. It's impossible."
Al-Assad said Syria is cooperating with the UN investigation into the killing but was concerned the inquiry would be politicised.
UN investigator Detlev Mehlis began investigating al-Hariri's killing two months after the bombing.
Four Lebanese generals have been arrested and charged with murder. Mehlis has said he has no Syrian suspects, though his team has questioned at least seven Syrians as witnesses.
The Syrian leader said he was ready to resume military and intelligence cooperation with the US on Iraq
provided it goes through a third party.
Syria, under increased US pressure over Iraq, ended military and intelligence cooperation with Washington in May due
to "unfair" US accusations that Damascus was doing too little
to stop foreign fighters from entering neighbouring Iraq.
"There has been an attempt to resume cooperation, basically,
through mediation by some Arab and European states," al-Assad said.
"If indeed there is a Syrian national implicated in it, he would be considered as a traitor and most severely punished"
"We said we have no objection, as long as it goes through a
third party. Now, those Arab and non-Arab parties went to say that to the US side, to say: 'What do you want from Syria'. So far, no response," al-Assad added.
He declined to speculate on whether the US military might attack Syria and whether Damascus would retaliate.
"We will deal with every situation if and when it happens," he said. "I cannot really go into hypothesis at this point. However, there is no such safe haven or camp of the kind to be bombed."
The US and Iraq argue that Syria could do more to close its borders to foreign fighters wanting to battle US-led forces in Iraq, but al-Assad says it is impossible for any country to fully safeguard its borders.