Speaking at the UN's headquarters in New York, John Bolton said, "Syria's record to date has been one of obstructing the investigation, of tampering with the evidence and not making witnesses available in a timely fashion."
Bolton's comments came after Syria confirmed it had received the UN commission's request to interview Bashar al-Assad, the president, and his foreign minister.
"The Security Council has made it clear that it expects full and unconditional compliance and said expressly in its resolutions that additional measures could be taken if need be," Bolton said. "We're trying to get the government of Syria to cooperate as it's required to do. The ball is in their court."
A Syrian official, who asked not be named because of the sensitivity of the investigation, said on Tuesday that the Foreign Ministry had received the request from the UN, which also asked to talk Farouk al-Sharaa, the foreign minister. The official did not comment on how Damascus planned to respond.Damaging revelations
The request for the interview with Assad came after former Abdul-Halim Khaddam, a former vice president, alleged last week that Assad had threatened al-Hariri several months before his assassination. An earlier request in July to interview al-Assad was refused.
The damaging revelations by Khaddam has further deepened suspicions of Syrian involvement in al-Hariri's killing, a charge that Syria denies.
Ahmad Hajj Ali, an analyst and member of Syria's ruling Baath party, said al-Assad could not be interviewed by the probe.
The UN inquiry wants to examine
allegations against al-Assad
"That's impossible because it would be an attack on [Syrian] sovereignty," said Hajj Ali.
"Firstly because there is no judicial pretext permitting an interview between the president and the commission and also because it would lead to a politicisation of the enquiry," he said.
Meanwhile, pan-Arab daily Al-Hayat reported that the inquiry's outgoing chief, German magistrate Detlev Mehlis, would hand over the reins of the enquiry to Belgian judge Serge Brammertz around 10 January.
In other developments, the father of Gebran Tueini, the slain anti-Syrian Lebanese lawmaker, said on Tuesday he would run for his son's seat in next month's by-elections, the official National News Agency reported.
Journalist-lawmaker Tueini was
killed by a car bomb in December
Ghassan Tueini, a distinguished journalist and a former senior diplomat and cabinet minister, said he had decided to run for parliament and vowed to fulfil his son's pledges to voters, the agency quoted him as saying in a statement.
The by-elections are set to take place on 5 February.
The younger Tueini, 48, a fierce critic of Syria, was killed along with his bodyguard and driver in a 12 December car bomb.
In May, he was elected to parliament in the first elections without Syrian military presence in Lebanon for nearly three decades.
Lebanon arrested a Syrian scrap dealer late in December in connection with Tueini's killing.