In their draft Security Council resolution, the three nations demand that “Syria must detain those Syrian official or individuals” implicated in the plot.

The draft threatens “further measures” — a reference to economic sanctions — if Syria fails to co-operate with the UN inquiry led by Detlev Mehlis, the German prosecutor.

It also calls for their assets to be frozen and a travel ban imposed on all individuals designated as suspects by Herr Mehlis’s investigation.

Herr Mehlis has found “converging evidence” of Syrian involvement in the St Valentine’s Day bomb blast that killed Mr Hariri, and implicated the Syrian President Bashar al-Assad’s brother, Maher, and his brother in-law, Assef Shawkat, in the plot.

If approved, the resolution could lead to travel and financial sanctions being imposed on members of President al- Assad’s family and inner circle.

Herr Mehlis has said publicly that a witness has linked Maher al-Assad and Mr Shawkat to the plot. But he refused to comment yesterday on whether the two men were considered “suspects”.

The draft also pointedly demands that Syria allow UN investigators to interview officials unaccompanied by government minders outside Syria.

The resolution is likely to come to a vote at a special Foreign Ministers’ meeting of the 15-nation Security Council on Monday. The final version may be watered down, however, by Russia and China, both veto-holding permanent members of the council.

The draft was circulated shortly after Herr Mehlis disclosed that his staff had received “credible” threats. He told the UN Security Council that he was concerned about the safety of his team after last week’s report implicating senior Syrian and Lebanese officials in the plot.

“The level of risk, which was already high, will increase further, particularly after the issuance of the report,” he said. “The commission has received a number of threats which were deemed, in the assessment of our security personnel, to be credible.”

Herr Mehlis said that the threats came from “unknown groups” and not specifically from any Syrian or Lebanese officials. “There were fliers that were being distributed in southern Lebanon threatening the commission and myself,” he said. “There were other more credible threats from alleged groups.”

Extra security measures have already been taken around the fortified UN offices in central Beirut, with additional concrete barriers installed. Even so, Nejib Friji, the UN spokesman in Beirut, had to be pulled out of the country “for his own safety” and has been temporarily reassigned to another UN mission.

President Bush said he hoped that Syria would co-operate with the UN inquiry. “I am hoping they will co-operate. (Military action) is the last — very last — option,” he told al-Arabiya television.

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