An increasingly isolated Syria today rejected as false, unprofessional and politicised an explosive UN report that accused Damascus of approving the assassination in February of Rafik Hariri, a former Lebanese prime minister.

The document almost certainly put Syria on a collision course with the UN Security Council, where the United States, Britain and France have been laying the groundwork for crippling economic sanctions against the regime of President Bashar al-Assad.

In Washington, Condoleezza Rice, the Secretary of State, telegraphed the Bush administration’s next move by declaring that Syria must be held accountable. "Accountability is going to be very important for the international community," she said.

While the UN findings did not directly implicate Mr al-Assad, the report cited a witness report that Assef Shawkat, the President's brother-in-law and Syria's military intelligence chief, forced a man to tape a claim of responsibility for Hariri’s killing 15 days before it occurred.

The report also implicated other Syrian officials in the massive bomb blast that killed Hariri and 20 others.

Hariri had quarreled with the country’s Syrian overlords and eventually resigned in October 2004, a month after Syria imposed a change in Lebanon’s laws to extend the term of President Emile Lahoud.

The pro-Syrian Mr Lahoud, meanwhile, denied a UN claim that he was one of two key officials who received a phone call minutes before the killing.

Mahdi Dakhlallah, the Syrian Information Minister, said the report was "a political statement directed against Syria". The report was based on witnesses "who are well known for their anti-Syria stands," Mr Dakhlallah’s office said, charging that the UN assessment lacked hard evidence and was based mainly on "gossip".

George Jabbour, a Syrian MP, said that the report by German prosecutor Detlev Mehlis was "extremely political" while Elias Murad, the editor-in-chief of the ruling party’s al-Baath newspaper, described it as a political rather than judicial report.

But Michel Kilo, a Syrian writer and political analyst who frequently criticises the Government, called on it to produce facts to counter evidence in the report. While the report contained some gaps, "I am convinced that it was professional," Mr Kilo said.

Herr Mehlis's report, submitted to the Security Council last night, implicated top Syrian and Lebanese intelligence officials in the assassination.

Hariri’s death sparked demonstrations against Syria and intensified the international pressure on Damascus to withdraw its troops from Lebanon, which it eventually did.

The report named two members of Mr al-Assad’s family as being part of the plot, citing testimony from an unnamed "witness of Syrian origin but resident in Lebanon, who claims to have worked for Syrian intelligence services in Lebanon".

Mr al-Assad’s youngest brother, Maher, and Mr Shawkat were part of a group of five senior Lebanese and Syrian officials who"decided to assassinate" the former premier "approximately two weeks after the [September 2004] adoption of Security Council resolution 1559," it said.

The report comes at a time of increased US pressure against Syria to stop interfering in Lebanon, to shut its border to anti-American insurgents crossing into Iraq and to desist in support for Palestinian militant groups. Syria has denied all of the accusations.

In one of the most critical parts of the UN report, Herr Mehlis said that Syria must co-operate if the continued investigation is to succeed. The probe, which was ordered by the Security Council on April 8, was extended for a second time by Kofi Annan, the UN Secretary-General, until December 15.

Earlier this week, a US official and two UN diplomats said that the United States and France were drafting Security Council resolutions critical of Syria for its alleged involvement in the Hariri assassination and alleged supply of arms to militias in Lebanon.

The 54-page document claimed that Mr Lahoud received a phone call minutes before the blast from the brother of a prominent member of a pro-Syrian group, who also called one of the four Lebanese generals, Raymond Azar, who have been arrested in the probe.

Mr Lahoud’s office issued a statement "categorically" denying that assertion. The statement said that the accusation was part of continued campaigns against the president "and the national responsibilities he shoulders and will continue to do so at this delicate stage in Lebanon’s history".

Since the arrest of the four Lebanese generals in August, anti-Syrian groups have focused on Mr Lahoud and demanded his resignation. He has refused to step down, saying his hands are clean.

Fuad Saniora, the Lebanese Prime Minister who is backed by the Hariri family and the anti-Syrian majority in Parliament, declined immediate comment on the report, saying he wanted to read it before convening Cabinet to discuss it.

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