The team was scheduled to interview the Syrians, whose names were not released, starting on 5 December and ending on 7 December in the Austrian capital, Ibrahim Gambari, the UN undersecretary-general for political affairs, said on Wednesday.
But diplomats, speaking on condition of anonymity, said later it appeared the head of the probe, German prosecutor Detlev Mehlis, would not go to Vienna to interview the five officials.
Instead he would send other investigators from his team. Gambari was not immediately available for comment.
Gambari also told a news conference he expected the investigation into the 14 February killing of former Lebanese prime minister Rafiq al-Hariri in Beirut to continue past its 15 December deadline "with or without Mehlis".
He said the prosecutor may have other commitments and would not want to serve indefinitely.
Mehlis is scheduled to submit his final report to the UN Security Council, which authorised the probe, on 15 December.
Syria may face sanctions if it does not cooperate with Mehlis.
Prosecutor Mehlis (L) may not go
to Vienna to interview the five
Gambari said time was short since interviews were only beginning next week, additional witnesses had to be questioned and more physical evidence needed to be analysed.
"The investigation has to continue," Gambari said. "It is very clear he will need an extension (and) the Lebanese government says it is now ready to approve an extension."
In Beirut, Lebanese Prime Minister Fouad Siniora said the government on Thursday would discuss whether to ask the Security Council to extend Mehlis' mandate.
He also said, without elaborating, that Lebanon was considering asking for an international trial for anyone who may be charged.
Secretary-General Kofi Annan will only recommend an extension to the UN Security Council after Mehlis issues his 15 December report, Gambari said.
Mehlis had originally wanted the interviews to take place in Lebanon but Syria balked and then compromised on a location in Vienna. Officials in Damascus also insisted those interviewed would be free to leave and not be detained.
Hussam Taher Hussam (L) asked
for his fiancee's protection
Meanwhile, Aljazeera's correspondent in Lebanon said Lebanese security authorities have detained the fiancee of Hussam Taher Hussam, a Syrian witness in the UN probe into al-Hariri's killing who has retracted his testimony against top officials in Syria.
Hussam earlier on Wednesday said that he feared for his fiancee Tharwat al-Hujayri.
"Hussam Taher Hussam says his fiancee in Lebanon is in danger and being put under pressure," his lawyer Omar Zohbi told a press conference in Damascus, standing next to Hussam.
He said Hussam was demanding that the Lebanese government protect Tharwat al-Hujayri, while his client appealed to UN Secretary-General Kofi Annan and human rights organisations.
Al-Hujayri was detained along with her father. The reasons of the arrests are not yet known, the Aljazeera correspondent said.
Hussam is a former intelligence agent whom the UN investigation into al-Hariri's murder in a 14 February bomb blast in Beirut quoted seven times in its interim report which implicates Syria.
The generals were detained on the
recommendation of a UN body
He recanted his testimony on Sunday.
He said on Syrian state television that he had testified against the brother and brother-in-law of President Bashar al-Assad only under duress and had been offered large bribes.
In another development, a former security chief detained in the investigation into al-Hariri's assassination was rushed to hospital with a heart problem on Wednesday, Lebanese security officials say.
Major-General Raymond Azar, the former chief of Lebanon's military intelligence, was taken from Roumieh prison to Hotel Dieu hospital in Beirut.
After emergency treatment, he was admitted to the intensive care unit, the officials said, speaking on customary condition of anonymity.
Azar is one of four Lebanese generals who were arrested in August on charges of being involved in the 14 February assassination of al-Hariri.
All the generals were pro-Syrian and had occupied senior positions in Lebanon's security agencies until being forced to step in the outcry that followed al-Hariri's murder.
Azar was admitted to hospital on
Wednesday for a heart problem
They were detained on the recommendation of the UN commission investigating the assassination, which issued an interim report last month that implicated the Syrian and Lebanese intelligence services.
The Lebanese government praised the report, but Syria rejected it.
The three other generals arrested in August are former General Security chief Major General Jamil Sayyed; former head of the Internal Security Forces, Major General Ali Hajj; and the commander of the Presidential Guard, Brigadier General Mustafa Hamdan.
Al-Hariri was killed in a truck bombing in central Beirut that killed 20 other people.
His death sparked a wave of mass protests which, combined with international pressure, forced Syria to withdraw its troops from Lebanon in April, ending a 29-year military presence in the country.