The head of the UN probe into the killing of ex-Lebanese Prime Minister, Rafik Hariri, has asked for more time to complete his inquiries.
Rafik Hariri had called for the withdrawal of Syrian troops
The UN investigator, Detlev Mehlis, told the Security Council at the current rate of progress his inquiry could take another year, or even two.
A UN report says Syrian intelligence officials were involved in the blast that killed Mr Hariri in February.
Syria's ambassador denied the findings, insisting Syria had co-operated fully.
Mr Hariri, who had called for the withdrawal of Syrian troops from Lebanon, was killed in a massive car bomb in Beirut in February.
In October, the Security Council called on Syria to co-operate with the inquiry, or face further action.
However, the BBC's Susannah Price, at the UN in New York, says there appears to be little appetite for imposing sanctions at the moment.
In his statement, Mr Mehlis said: "It has been after much hesitation and procrastination that the Syrian authorities finally agreed to move on the request to interview five Syrian officials - whom the commission considers as suspects - in Vienna under conditions determined by the commission.
"This took place only a week ago. At this rate the investigation may take another year or two."
In response, the Syrian Ambassador to the UN, Fayssal Mekdad, insisted his country had made every possible effort to help the investigation.
"We are ready to do whatever the commission requests us to do," he told the UN Security Council.
Mr Mekdad criticised the investigators for not keeping their findings private, and said they had ignored basic rules of human rights by making some suspects sign statements in a language they did not understand.
He later told journalists that Syria had "nothing to do" with what he termed a "heinous crime".
"We believe that this investigation will lead to the clearance of Syria because it is in our interest to find the truth," Mr Mekdad said.
The Lebanese ambassador said he hoped that the Security Council would extend the mandate of the UN investigation for a further six months.
Our correspondent says the Security Council is expected to approve this request.
Before the debate began, the US ambassador to the UN, John Bolton, told reporters that Syria had already ignored many UN resolutions.
"The government of Syria responds unfortunately only to pressure - at least that's our experience to date - so we are considering what additional pressure we will bring to bear...
"Their lack of co-operation - indeed their affirmative obstruction of the Mehlis commission - needs a response and we're looking to find exactly what that should be."
The Security Council meeting was adjourned for further discussions without a vote.
Wave of killings
Mr Hariri and 20 other people died in the 14 February attack, when a huge bomb exploded as Mr Hariri's convoy drove through central Beirut.
The attack led to widespread protests in Lebanon and extensive international pressure on Syria, forcing it to end its 29-year-old military presence in Lebanon in April.
Gibran Tueni was a fierce critic of Syria's occupation of Lebanon
The initial UN inquiry report said it was highly unlikely that the attack could have been carried out without Syria's knowledge.
Since the assassination, at least 14 bomb attacks have been carried out against Christian and anti-Syrian targets.
The latest, on Monday, saw the prominent anti-Syrian journalist, Gibran Tueni, killed in a car bomb explosion.
This has deepened the political crisis in Lebanon.
Several pro-Syrian ministers have said they are suspending their participation in the government after Prime Minister Fouad Siniora called on the UN to widen the Hariri inquiry to look into the wave of killings of prominent anti-Syrian figures.