Briefing the Security Council on his latest findings regarding the February murder of Mr Hariri, the German prosecutor noted that "after much hesitation and procrastination" Syrian authorities finally agreed to let his team interview five Syrian suspects in Vienna last week.
"This latest development is undoubtedly an important stage in the investigation," he noted.
However, he said that because of the slow pace of the Syrian cooperation, "the investigation might take another year or two."
He stressed that under normal circumstances "it should be much faster."
"It was not clear at all times who from the Syrian side is the privileged interlocutor of the commission. This has caused confusion and delays," Mr Mehlis said.
He said extensive interviews of the Syrian suspects in Vienna had to be assessed and new evidence be reviewed before leading to questioning of new witnesses or suspects.
Mr Mehlis expressed hope that the interviews could be the "starting point" of greater Syrian government cooperation with the commission.
Asked to assess the level of Syrian co-operation, he told reporters: "We are not seeing full cooperation. Hopefully it will turn into full co-operation."
Differing views
But Syria's UN envoy Fayssal Mekdad insisted that his country "has co-operated fully during the last period and reiterates its readiness to co-operate with the investigation during the upcoming period."
That view was not shared by US ambassador to the UN John Bolton, who said: "Syrian cooperation has been grudging at best".
Mr Bolton said that Damascus had failed to comply with a Security Council resolution last October 31 which warned Syria to fully cooperate with the UN probe or face possible sanctions.
"We are looking for ways to make sure that the international pressure on Syria is unrelenting," Mr Bolton said.
"The Council's word is at stake now... For the Council's credibility to be preserved it has to ensure that the pressure and compliance continue," he said.
"On the part of United States there is absolutely no wavering from the proposition that Syria is not going to get away with obstructing this investigation, it's not going to cover up the actions of its senior officials and it's not going to escape the consequences."
Mr Mekdad replied: "Ambassador Bolton has always been wrong. He was saying that Iraq had weapons of mass destruction and he was wrong... He is always wrong and this is my answer to his allegations."
The Mehlis report, which cited fresh evidence of Syrian involvement in the Hariri murder, was delivered to the Security Council Monday only hours after a car bomb in Beirut killed prominent anti-Syrian lawmaker Gibran Tueni, the latest attack against a Damascus critic in Lebanon.
In his report, Mr Mehlis said two of the five Syrian suspects questioned in Vienna indicated that all Syrian intelligence documents concerning Lebanon had been burned.