UN investigators have been questioning five Syrian officials as part of their inquiry into the killing of former Lebanese Prime Minister Rafik Hariri.
Rafik Hariri had been at a session of parliament just before the blast
The officials, who have not been identified, were questioned at United Nations offices in Vienna, Austria.
An interim report by investigator Detlev Mehlis has already implicated Syrian and pro-Syrian Lebanese officials into Hariri's death.
Damascus has strongly denied involvement in the assassination.
It agreed to let the interrogations of Syrian officials take place abroad after assurances that they will be able to return to Damascus afterwards, according to diplomatic sources.
The questioning is expected to end on Wednesday.
The UN Security Council has warned Syria it faces unspecified action unless it co-operates fully with the investigation.
Mr Mehlis is due to submit another report to the Security Council on 15 December, but the investigation is likely to continue beyond that date.
Lebanon has urged UN Secretary General Kofi Annan to extend the investigation by six months.
Last week, a Syrian witness retracted testimony implicating Syrian officials, saying he had been bribed and tortured by the Lebanese authorities to give evidence against Syria.
INTERIM UN FINDINGS
Assassins had considerable resources and capabilities
Evidence suggests both Syria and Lebanon were involved
Crime was prepared over several months
Hariri's movements and itineraries were monitored
Highly unlikely Syrian or Lebanese intelligence were not aware of assassination plot
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Hossam Taher Hossam, a Syrian intelligence agent once based in Lebanon and one of the witnesses in the investigation, appeared on Syrian TV to retract testimony to the UN in which he mentioned a meeting at the Syrian presidential palace to plot the assassination.
Syria has demanded that the interim UN report that was released in October be modified accordingly.
"We expect this inquiry to be professional and the commission to correct the mistakes it made earlier in order to form a report that is fair and objective," Syrian President Bashar al-Assad told France 3 television.
BBC Beirut correspondent Kim Ghattas says the anti-Syrian political majority in Lebanon is eager to see Damascus implicated in the murder, but a lot in Mr Hossam's statements simply did not add up.
The chief UN investigator has dismissed the whole affair as Soviet-style propaganda by the Syrians.
Mr Mehlis insists that Mr Hossam was just one of many witnesses, and that his investigation is on the right track.