UN investigators started questioning five Syrian officials in Vienna on Monday over the assassination of former Lebanese prime minister Rafik al-Hariri in Beirut, diplomatic sources said.
Syria, which denies any role in the killing, agreed to let investigators quiz the men at UN offices in the Austrian capital after getting guarantees from permanent UN Security Council member Russia that they could return to Damascus afterwards, diplomats said.
"This is the case," a diplomatic source told Reuters when asked if the questioning had started, declining further comment.
The Security Council has warned Syria to cooperate with the probe or face unspecified action that could lead to sanctions.
An interim report issued in October by the chief UN investigator, Detlev Mehlis, suggested the February 14 truck bombing that killed Hariri and 22 other people was planned by top Syrian security officials in Damascus and their Lebanese allies.
Diplomats said the five officials being questioned included Syria's former intelligence chief in Lebanon, Lieutenant General Rustom Ghazali, before Damascus pulled forces from its small neighbour in April after 29 years.
One diplomatic source named the others as Lieutenant General Thafer Youssef, Lieutenant General Abdul-Karim Abbas, and a Ghazali aide, Jamea Jamea. The fifth was a civilian official, a Syrian source said.
Diplomats said the questioning at the UN complex in Vienna was expected to be carried out between December 5 and 7.
They said Mehlis could ask for the arrest of some of the five after they return home. Syria's reaction could determine whether Mehlis eventually declares it in breach of a Security Council resolution demanding co-operation with the investigation.
UN investigators have interviewed more than 500 people over the killing. On Mehlis's recommendation, Lebanon has also charged with murder four pro-Syrian Lebanese generals.
Lebanon's government has asked UN Secretary-General Kofi Annan to extend the mandate for the inquiry for six months beyond the December 15 deadline and be open to further extensions.
Mehlis, a German prosecutor, has said his team might ask to question more Syrian officials.
The assassination of Hariri, a strong opponent of Syrian control over Lebanon, stirred an international outcry and weeks of Lebanese street protests that brought about Syria's pullout. It had sent forces into Lebanon in 1976 to quell a civil war.
Syria has dismissed the suspicions as politically motivated. But analysts say President Bashar al-Assad faces a dilemma: rein in powerful figures in Syria's security apparatus whom he has never dominated as his late father Hafez al-Assad did, or do nothing and invite a ruinous showdown with the United Nations.