Washington is trying to arrange a quick, high-level UN Security Council meeting to consider a response to the inquiry that named senior Syrian officials as suspects in the February truck bombing that killed al-Hariri and 22 others.
US Secretary of State Condoleezza Rice and British Foreign Secretary Jack Straw told the BBC in a joint interview on Sunday that the investigation strongly implicated Syria and indicated attempts at a cover-up.
"I am quite sure that when the international community gets together, we will decide what to do, but it can't be ... left lying on the table," Rice said. "This really has to be dealt with."
The UN report found last week that the decision to kill al-Hariri "could not have been taken without the approval of top-ranked Syrian security officials" colluding with officials in Lebanon.
It named senior Syrian security officials, including Syrian President Bashar al-Assad's brother and brother-in-law, and their Lebanese allies, as suspects in the murder that transformed Lebanon's political landscape.
Syrian officials have dismissed the report as political and said the charges were false but left the door open for future cooperation with the inquiry, saying it might agree to allow investigators to return to Damascus to question Syrian officials.
The report named Bashar
al-Assad's relatives as suspects
Straw said: "The report indicates that people of a high level of this Syrian regime were implicated."
He added: "We also have evidence from the ... report of false testimony being given by senior people in the regime. This is very serious."
Straw said on Friday that UN Security Council members would consider sanctions but acknowledged the West had to work to win support from all members to put pressure on Damascus.
The report said the Syrian authorities, after initially hesitating to help, had cooperated "to a limited degree", but several individuals had tried to mislead investigators "by giving false or inaccurate statements".
Walid al-Mualim denied he had
threatened Rafiq al-Hariri
Syrian Deputy Foreign Minister Walid al-Mualim denied on Sunday accusations in the report that he had threatened al-Hariri two weeks before his assassination.
"This is completely untrue. I did not go to prime minister Hariri to make threats," he said. "I went to tell him about my mission and ask him to cooperate in order for the mission to succeed," he told Syrian state television, without elaborating.
The report said al-Mualim had avoided giving direct answers to questions and lied to investigators when he described a meeting with al-Hariri on 1 February as "friendly and constructive".
Jumblatt urges cooperation
It includes part of a recording of the meeting in which al-Mualim told al-Hariri: "We and the [security] services here have put you into a corner" and "please do not take things lightly".
Lebanon's Druze leader Walid Jumblatt, a prominent critic of Syria, urged al-Assad on Sunday to cooperate with the inquiry and backed a call from Saad al-Hariri, the son of the slain former prime minister, for an international tribunal to try the suspects.
"If I have any advice to give to the Syrian president it is to cooperate for the sake of the investigation or for the sake of uncovering the truth," said Jumblatt, at the forefront of a campaign that forced Syria to relinquish its domination of Lebanon.
Jumblatt urged Bashar al-Assad
to cooperate with the inquiry
"Our aim is for the investigation to continue on the domestic Lebanese side and internationally because the Lebanese courts, under the agreements between Lebanon and Syria, cannot question Syrian suspects in Lebanon," he said.
"If necessary we will seek the establishment of an international trial."
UN Secretary-General Kofi Annan has extended the investigation's mandate, which suggests no strong action will be taken until it ends on 15 December.
Al-Assad told CNN in an interview a week before the UN report was published that he could not have ordered the killing. He
said the involvement of any Syrian would be considered treason and would be punished in Syria or internationally.
In the first arrest since the report was released, a suspect accused of calling pro-Syrian Lebanese President Emile Lahoud minutes before the murder was detained over the weekend.
Security sources said Mahmud Abd al-Al was detained based on a warrant from the public prosecutor although Lahoud's office has strongly denied the president had any contact with him.
In the months ahead of his murder, Rafiq al-Hariri had joined calls for an end to Syria's domination of Lebanon. Many Lebanese have long blamed Syria and its local allies for his death.