Led by veteran German prosecutor Detlev Mehlis, the probe into the 14 February assassination cited "converging evidence" of Syrian and Lebanese involvement and accused Damascus of blocking and misleading the investigation.
"There is probable cause to believe that the decision to assassinate ... could not have been taken without the approval of top-ranked Syrian security officials and could not have been further organised without the collusion of their counterparts in the Lebanese security services," the report said.
Reacting to the report on Friday after helping dedicate a new pavilion at the Ronald Reagan Presidential Library and Museum in Southern California, Bush said:"The report strongly suggests that the politically motivated assassination could not have taken place without Syrian involvement."
Bush said he instructed Secretary of State Condoleezza Rice earlier in the day to call upon the UN to convene a session "as quickly as possible to deal with this very serious matter".
Bush was not specific about what steps the international community should take to make sure Syria is held accountable.
He said the US had started talking with UN officials and with Arab governments about what steps to take.
"Today a serious report came out that requires the world to look at very carefully and respond accordingly," Bush said.
Earlier on Friday, Rice said she was deeply troubled by the UN report, adding that the international community had to find a way to hold Syrian authorities accountable.
Rice: Accountability is going to
be very important for the world
In Washington, another top State Department official said al-Hariri was the victim of a "political crime" that could not have been carried out without the involvement of senior Syrian and Lebanese intelligence officials.
Assistant Secretary of State C David Welch said in Washington that "we would like to see those responsible for this crime and others in Lebanon brought to justice".
The UN's exhaustive report linked the brother-in-law of Syria's president to the 14 February car bomb that killed al-Hariri and 20 others, and said Lebanese intelligence officials helped organise it.
The report stopped short of fingering Syrian President Bashar al-Assad or his inner circle, but it accused the government of failing to cooperate in the inquiry.
The report also alleged Syrian Foreign Minister Farouk al-Sharaa lied in a letter to the investigating commission.
Rice declined to discuss next steps beyond saying that some kind of international mechanism must be established to ensure that Syria is held accountable.
She said there was strong support among UN members for an extension of Mehlis' mandate, perhaps until 15 December.
Welch, speaking at the Foreign Press Centre, said the Bush administration had begun discussions at the UN and with Arab and other governments on how to act on the report.
He said some Arab governments share the administration's concern about Syria's "destabilising" actions in Lebanon, but he declined to identify with whom the US was finding initial support.
Welch, who said he had read the report, said it contained "amazing evidence".
"The report concludes there is probable cause to believe the [assassination] decision could not have been taken without the collusion of top Syrian and Lebanese intelligence officials," Welch said.
Syrians are worried about the
possibility of UN sanctions
For her part, Rice said: "Even an initial reading of the report is deeply troubling.
"You have clearly a case in which there is an implication that Syrian officials were involved in the assassination of Rafiq al-Hariri.
"You also have a clear indication that the Syrian government has not been cooperating."
She added: "These are charges that will lead the international community to have to seriously consider how it demands accountability."
Speaking to the media on Friday, Syria's ambassador to the UN, Faisal Mekdad, said: "All the charges made against Syria are categorically void of the truth."
Saying that the late al-Hariri "had been a close friend to Syria", he said "as such it was not in the interest of any party to commit such a folly".
Mekdad added: "It would also be a big mistake to punish states that do not threaten world peace and security."