Assad's announcement followed charges last month from U.S. Secretary of State Condoleezza Rice and French President Jacques Chirac that the Damascus seated government was hindering the U.N. inquiry by refusing to cooperate with lead investigator Detlev Mehlis.
A Security Council resolution in October also urged the country to cooperate with the investigation or face international action.
In a nationally televised speech from Damascus University, Assad said he believed the investigation and a subsequent draft report naming top Syrian and Lebanese officials as suspects in Hariri's death, to be part of an international effort to pressure Syria into submission.
"No matter what we do and how much we cooperate, the result after a month will be that 'Syria did not cooperate' ... but we have to do our duty," he told thousands of flag-waving young Syrians gathered at the university, according to BBC reports. "We support international legitimacy but not at the expense of our national interests."
Syria has repeatedly denied a role in Hariri's murder, but also has refused a request from Mehlis to question Syrian suspects across the border in Lebanon.
Instead, Assad blasted the Lebanese government for supporting a conspiracy against his country. He said Lebanese Prime Minister Fouad Siniora had allowed Lebanon to become a base for Syrian enemies.
"The truth we see today is that Lebanon has become a passageway, a factory and a financier of these conspiracies," he said.
Many Lebanese believe Syria had a hand in Hariri's death. On Thursday, Druze leader Walid Jumblatt, a Hariri ally and critic of the Syrian government, criticized Assad's comments.
"This is unbecoming of the president of Syria," he told Reuters. "All of us, the successors of Rafik al-Hariri and myself, insist on knowing the truth behind this murder."
According to Assad, Mehlis has refused an invitation to question the suspects, including his brother and brother-in-law, at U.N. headquarters in Syria, Reuters reported.
Asked whether the U.N. would begin drawing up a new resolution against Syria, Rice urged the Syrian government to cooperate unconditionally, Reuters reported.
"They should stop trying to negotiate and cooperate," Rice said. "They are expected to answer affirmatively, positively, yes to whatever Mehlis needs to complete his investigation. I do not believe the [U.N. Security Council] resolution contemplated the Syrians negotiating how they would say yes."
A final report from Mehlis is expected Dec. 15. Syrian officials are also in the process of conducting a separate investigation into Hariri's death.
-- Compiled from wire reports and other media sources