Husam Taher Husam, a former conscript in the Syrian army, alleged in a 75-minute interview on Sunday night that Saad Hariri, the son of the slain Hariri, met him several months ago and offered him $1.3m to testify against top Syrian officials.

The spokesperson for the Syrian inquiry into Hariri's murder, Ibrahim Daraji, said on Monday that if Husam is the unidentified key witness quoted in the UN commission's interim report, then the United Nations' case "has completely collapsed."

Daraji spoke at a press conference in Damascus on Monday at which Husam reiterated the allegations he had made on Syrian television the night before.

Syria criticised UN report

Husam told the television that UN officials told him what to say when he gave evidence to the UN commission, in particular that he was "close to" Brigadier General Assef Shawkat, the chief of Syrian military intelligence and brother-in-law of Syria's president, who was named in the commission's interim report last month.

"But I've never seen him in my whole life," Husam said of Shawkat in the television interview.

It was not possibly to reach Saad Hariri for a response on Monday as he was travelling in South America. News bulletins on Hariri's own Future TV station did not refer to Husam's claims. The UN commission rarely responds to media reports about the investigation.

Husam's allegations came days before five senior Syrian officials were due to appear before the UN commission in Vienna. The officials, who have not been named either by the commission or Syria, will be questioned in the UN headquarters in the Austrian capital as part of an agreement reached after more than two weeks of negotiations over where and how their evidence would be taken.

In its interim report, the commission implicated the Syrian and Lebanese intelligence services in the Beirut bombing that killed Hariri and 20 others on February 14. Lebanon welcomed the report, but Syria rejected it as politicised and unfounded on evidence. Syrian officials have for weeks tried to discredit the UN investigation as biased against Syria.

Previously, another Syrian, Mohammed Zuhair Siddiq, gave evidence to the commission but was later discredited. At the commission's recommendation, he was arrested in France in October and extradited to Lebanon where he is being held as a suspect in the murder.

Hariri's assassination, which many Lebanese blame on Syria, was the catalyst for mass anti-Syrian street protests and intensified international pressure that forced Syria to withdraw its army from Lebanon, ending nearly three decades of domination.