UNITED NATIONS - The chief U.N. investigator into the assassination of a former Lebanese prime minister is expected to begin questioning Syrian witnesses in Vienna next week, U.N. diplomats said on Wednesday.

German prosecutor Detlev Mehlis and his team were expected to interview the five Syrians, whose names were not released, between December 5 and December 7 at U.N. headquarters in the Austrian capital, according to two sources familiar with arrangements for the talks. The envoys spoke on condition of anonymity.

Syria last week agreed to allow five officials to leave Damascus for questioning by the team investigating the killing of former Lebanese prime minister, Rafik Hariri, in Beirut last February 14.

Mehlis had originally wanted the interviews to take place in Lebanon but Syria compromised on a location in Vienna. Officials in Damascus also insisted the deal for the interviews exclude any arrests or detention of those questioned.

Mehlis was authorized by the U.N. Security Council to probe the murder of Hariri and was expected to release his final report on December 15, although the council could extend his work.

In Beirut, Lebanese Prime Minister Foaud Siniora said the government would on Thursday discuss whether to ask the Security Council to extend Mehlis's mandate and to establish an international trial for anyone who may be charged.

He gave no further details.

Mehlis' interim report in October cast suspicion on senior Syrian officials and suggested the assassination was planned by top security officials in Damascus and their Lebanese allies.

His report also criticized Syria for failing to cooperate with the investigation, including having officials present when his team questioned witnesses and suspects. The Security Council subsequently warned Syria to cooperate or face the prospect of further action, which could lead to sanctions.

Syria has denied the accusations and called the Mehlis report politically motivated. But in an apparent effort to avoid a confrontation, Damascus agreed to the interviews in Vienna.

The death of Hariri, a strong opponent of Syrian domination of Lebanon, transformed Lebanon's political landscape. The killing led Syria to pull its troops from Lebanon after three decades. Damascus had gone into Lebanon to quell a civil war.

Lebanese political sources say Mehlis had asked to question six officials, including Syrian President Bashar al-Assad's brother-in-law, Major-General Assef Shawkat, head of military intelligence, and Major-General Bahjat Suleiman, former head of the internal security branch in the general intelligence department.

However, more junior officials were expected to be among the five due to be interviewed next week. Sources identified them as Lieutenant-General Rustom Ghazali, Lieutenant-General Thafer Youssef, Lieutenant-General Abdul-Karim Abbas and an aide to Ghazali. (Reuters)