Lebanese Prime Minister Fouad Siniora has urged all sides to co-operate with a UN team investigating the killing of the former Prime Minister Rafik Hariri.
UN investigators may ask for more time to look into Hariri's death
Mr Siniora - who was speaking after the UN accused Syria of withholding documents and shielding witnesses - did not mention Syria by name.
But he said the Lebanese needed to know the truth about Mr Hariri killing.
Syria's representative at the UN, Faisal Meqdad, denied that Damascus was being obstructive.
Mr Hariri died in February in a blast many of his supporters blamed on Syria.
Massive street protests followed his death, which - coupled with international pressure - led Syria to end a 29-year troop deployment in Lebanon.
Elections following the withdrawal brought to power an anti-Syrian coalition headed by Mr Hariri's son, Saad.
Failure to respond
The UN investigation into Mr Hariri's death - headed by German prosecutor Detlev Mehlis - has a mandate that expires in mid-September, though many diplomats expect this to be extended.
Syria has failed to respond to a request from the investigators to provide key documents and five witnesses for interview, UN Under Secretary General Ibrahim Gambari said.
"No reply has been received," he said.
He noted that Syria had offered to discuss the investigators' demands - but he said this was no substitute for co-operation.
The US envoy to the UN, John Bolton, said Mr Gambari's briefing made it clear that "Syria's lack of co-operation with the Mehlis investigation has considerably slowed down" its work.
Mr Bolton also said he wished the UN Security Council statement that followed Mr Gambari's briefing had been more direct in its criticism of Syria.
"We are disappointed that we couldn't be clearer today in the council.
"Let there be no ambiguity about the American view that Syria's lack of co-operation is not acceptable," he said.
Damascus was not mentioned by name in the council's statement, which called on "all states and all parties, especially those who are yet to respond adequately, to co-operate fully in order to expedite the work of the [inquiry] commission".
Diplomatic sources say Russia and Algeria had demanded the council drop any direct criticisms of Syria from the statement.