Condoleezza Rice, the US secretary of state, issued a stern new warning on Wednesday to the Damascus government as Belgian prosecutor Serge Brammertz took charge of the UN investigation into the assassination of al-Hariri in Beirut in February 2005.
Rice launched a broad attack on Syria, accusing it of creating an "atmosphere of fear" in Lebanon to maintain its influence in its small neighbour.
"The United States has grave and continuing concerns about Syria's destabilising behaviour and sponsorship of terrorism," she said in a statement.
Rice said Syria has "failed" to implement five UN Security Council resolutions over the assassination.
"Syria must cease obstructing the investigation into the assassination of former Lebanese prime minister Hariri and instead cooperate fully and unconditionally, as required by UN Security Council resolutions."
Rice continued: "We call upon the Syrian regime to respond positively to the requests of UN Independent International Investigation (UNIIIC). We intend to refer this matter back to the Security Council if Syrian obstruction continues.
"The United States stands firmly with the people of Lebanon in rejecting any deals or compromises that would undermine the UNIIC investigation, or relieve Syria of its obligations under UN Security Council resolutions.
"We are firmly committed to seeking justice and pursuing the investigation to its ultimate conclusion."
Rice mentioned Gebran Tueni's
killing in her speech on Syria
The UN investigation has already implicated Syrian officials in the killing but this has been strongly denied by the Damascus government.
Also on Wednesday, Kofi Annan, the UN secretary-general, named Serge Brammertz, 43, as the new head of UNIIIC after a meeting with the Belgian prosecutor.
Brammertz, currently deputy prosecutor at the International Criminal Court (ICC), replaces German investigator Detlev Mehlis in charge of the inquiry.
A UN spokesman said Brammertz would head to Beirut "as soon as is practicable" and that Annan would soon send a mission to Lebanon to help authorities "identify the nature and scope of the international assistance needed for those charged with the crime to be tried by a tribunal of an international character".
Mehlis has released two reports on the murder in Beirut, implicating Syrian and Lebanese intelligence officers and casting doubt on Damascus' cooperation with the UN probe.
Mehlis said Syria was behind
al- Hariri's murder last year
In an interview with a Lebanese newspaper last month, he unequivocally accused Syria of being behind the car bombing that killed al-Hariri last year.
Last month, the Security Council passed a resolution endorsing a six-month extension of the murder probe and renewing its call for Syria's full cooperation with the investigation.
The resolution also authorised technical assistance to Lebanon to help authorities probe recent murders of anti-Syrian politicians.
The other murders were referred to by Rice when she accused Syria of seeking to maintain its influence in Lebanon.
Rice said: "Continuing assassinations in Lebanon of opponents of Syrian domination, including most recently the murder of journalist and member of parliament Gebran Tueni on December 12, 2005, create an atmosphere of fear that Syria uses to intimidate Lebanon.
"Syria must cease this intimidation and immediately come into compliance with all relevant Security Council resolutions," she said.
"The United States stands firmly with the people of Lebanon in rejecting any deals or compromises that would undermine the UNIIC investigation"
US Secretary of State
Rice also reaffirmed calls for the implementation of UN resolution 1559, which includes the disarmament and disbanding of Hizb Allah and other militias.
"Syria's continuing provision of arms and other support to Hizb Allah and Palestinian terrorist groups serves to destabilise Lebanon, makes possible terrorist attacks within Lebanon, from Lebanese territory, and impedes the full implementation of Security Council resolutions," she said.
"As Resolution 1559 demands, Syria must once and for all end its interference in the internal affairs of Lebanon."