A UN inquiry into the assassination of former Lebanese Prime Minister Rafik Hariri says many leads point to the direct involvement of Syrian officials.
There are fears the report could add to Lebanon's political turmoil
UN investigators said they had also found evidence of Lebanese collusion in Mr Hariri's death last February.
The report, prepared at the request of UN Secretary General Kofi Annan, says the car bombing was carried out by a group with extensive organisation.
Both Syria and Lebanon have denied the allegations of official involvement.
Syria's Information Minister Mehdi Dakhlallah condemned the findings as "politically biased" deceptions and said the report was "far from the truth".
The Lebanese presidency issued a statement denying that a suspect implicated in Hariri's assassination had called President Emile Lahoud minutes before the truck bomb exploded.
KEY UN FINDINGS
Assassins had considerable resources and capabilities
Evidence suggests both Syria and Lebanon were involved
Crime was prepared over several months
Hariri's movements and itineraries were monitored
Highly unlikely Syrian or Lebanese intelligence were not aware of assassination plot
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"The press office in the presidential palace categorically denies this information, which has no basis in truth and is a part of a pressure campaign against the president," it said.
A spokesman for the US state department meanwhile said the report was "deeply troubling".
Many in Lebanon blamed Syria for the killing, a claim denied by Damascus.
Troops have been deployed in the Lebanese capital, Beirut, and other cities in case of violence in the wake of the inquiry's findings.
The UN team's report said there was "converging evidence pointing at both Lebanese and Syrian involvement in this terrorist act".
Mr Hariri's assassination - for which the likely motive was political - was complex and had obviously been planned for months, the report says.
As such, it "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 inquiry established that "many leads point directly towards Syrian security officials as being involved with the assassination".
The investigators, led by German magistrate Detlev Mehlis, say the Syrian authorities co-operated only to a degree and accuse several interviewees of "trying to mislead the investigation".
They also say a letter from Syrian Foreign Minister Farouq al-Shara to the commission contained "false information".
The investigation is not complete, the report said, with several lines of inquiry still to be pursued.
The UN Security Council will be briefed on 25 October by Mr Mehlis, who handed over the report to UN Secretary General Kofi Annan on Thursday.
John Bolton, the US ambassador to the UN, said several countries had been consulting on what steps to take based on the report's findings.
Hariri and 20 others died in the car bomb attack on 14 February
"We're going to study it very carefully, and based on what's in there, decide what our next course of action might be," he said.
The BBC's Ian Pannell in Damascus says it is widely believed that the Syrian authorities were angered by Mr Hariri's growing opposition to their influence in Lebanon.
Although Syria's President Bashar al-Assad constantly denies any involvement in the killing, the report will add to the sense of impending crisis felt in the country, our correspondent says.
Syria finds itself almost completely isolated, with little support from other Arab nations, and faces the prospect of crippling UN sanctions, he adds.
Syria was the main power in Lebanon until its military withdrawal earlier this year.
Since Mr Hariri was killed on 14 February, a series of bomb attacks have targeted anti-Syrian journalists and politicians as well as Christian areas in Lebanon.
Mr Hariri's son, Saad, has said he wants those implicated by Mr Mehlis' report to be tried by an international court.