Detlev Mehlis will also probably ask for more time than the designated three months to complete his findings, the official said.
"Detlev Mehlis needs to directly interview Syrian officials concerned. He needs to visit Syria for this purpose," UN spokesman Najib Friji said on Saturday.
"The Syrians have agreed in principle to cooperate with Mehlis but he has yet to receive an official Syrian response to visit the country."
It was not clear which Syrian officials Mehlis plans to question, although he announced at the start of the inquiry that "we will ... investigate anyone who was in one way or another responsible for security in Lebanon at the time of the crime".
Lebanon's As-Safir newspaper reported on Saturday that Mehlis had already questioned three Syrian officials in writing rather than in person after Damascus declined direct interviews.
Al-Hariri's killing was a turning
point in the country's politics
"In his report, Mr Mehlis will request an extension of his mission," Friji said. He declined to say how much more time Mehlis will ask the world body to grant him.
But Lebanese Prime Minister Fuad Siniora said on Friday the UN mission may need a few weeks only beyond its 15 September deadline to complete the investigation.
Siniora spoke after two hours of talks with Mehlis.
"He gave me some ideas, but there is nothing specific," Siniora said afterwards, referring to the investigation. Mehlis, a German prosecutor, did not talk to reporters.
The UN Security Council voted in April to authorise Mehlis' probe after a UN fact-finding team concluded that a Lebanese investigation into the killing did not meet international standards.
"The Syrians have agreed in principle to cooperate with Mehlis but he has yet to receive an official Syrian response to visit the country"
The UN team has 30 investigators, including an explosives unit from Germany, crime technicians from the Netherlands, and divers from Britain - the bomb exploded next to the Beirut waterfront.
So far, the team has interviewed 68 witnesses and one suspect. It has also set up hotlines for information and contacted Interpol for technical assistance.
Al-Hariri's assassination was a watershed in Lebanese politics. It triggered mass protests that brought down the pro-Syrian government two weeks later, and it greatly increased international pressure on Syria to withdraw its troops from Lebanon. Syria withdrew the last of its troops in late April.
Lebanese opponents of Syria's role in Lebanon accused Syria and its allies in the Lebanese security services of involvement in al-Hariri's killing. Both authorities denied any involvement.
On Saturday, 10 UN investigators returned to the scene of the explosion, the seafront boulevard near the Saint Georges' Hotel. One investigator was seen carrying a paper bag of material he had apparently picked up at the spot.
Mehlis has said little about the progress of the investigation. But in June he gave a press conference in which he said the explosives were probably detonated in a truck.
Many Lebanese had speculated the bomb was buried under the road, which would suggest official involvement as digging under a main road would require a permit.
"We are talking here about a probability of 99.9 percent" of an above-ground explosion, he said.