The 25-page report from the team of Detlev Mehlis, the German prosecutor, again accused Syria of trying to obstruct his inquiry when it demanded that he revise his findings after a key witness recanted his testimony.
The report was delivered on the same day that a car bomb killed a prominent Lebanese journalist and lawmaker, Jebran Tueni.
It was the latest in a string of assassinations of anti-Syrian figures in Lebanon, and many quickly accused Damascus of the slaying.
Mehlis's team delivered a report in October that implicated Syrian and Lebanese security officials in Rafiq al-Hariri's assassination in February, a car bombing that killed 22 other people. Mehlis said greater Syrian co-operation was needed.
Syria denies involvement in the assassination.
It has also tried to discredit the commission, citing a Syrian witness, Hussam Hussam, who retracted his testimony to the commission and claimed to have been bribed to frame Syria.
Mehlis said that recantation had not affected his findings.
Hussam Hussam (L) retracted
an earlier statement on Syria
He said: "The investigation has continued to develop multiple lines of inquiry which, if anything, reinforce those conclusions."
The latest claim of obstruction would be important because after Mehlis delivered his earlier report, the council had warned Syria that it would face further action - possibly including sanctions - if it did not co-operate fully.
The report also said Syrian co-operation with its investigation was slow and called on Damascus to arrest Syrian suspects.
"Given that its substantive lines of inquiry are far from being completed, and given the slow pace with which the Syrian authorities are beginning to discharge their commitments ... the commission recommends that there be such an extension and for a minimum period of six months," it said in a copy of the report to the Security Council obtained by Reuters in Beirut.
"Syria must detain those Syrian officials or individuals whom the commission considers as suspected of involvement in the planning, sponsoring, organising or perpetrating of this terrorist act"
"Syria must detain those Syrian officials or individuals whom the commission considers as suspected of involvement in the planning, sponsoring, organising or perpetrating of this terrorist act, and make them fully available to the commission."
The report said it had identified 19 suspects but did not name them. It said five Syrian officials questioned by UN investigators in Vienna this month were suspects.
A Syrian official, who declined to be named, said: "The report will be studied in all its legal and political aspects to take the decisions that are essential and that will serve the interests of Syria."
He said that his country would rather wait until a meeting of the Security Council on Tuesday before issuing a full reaction, but he said: "There is nothing that justifies measures against Syria."
Last week, members of the Mehlis commission questioned several senior Syrian officials at the UN headquarters in Vienna. UN diplomats there said Rustum Ghazali, the last Syrian intelligence chief in Lebanon who was in charge when al-Hariri was assassinated, was among them.
Mehlis will brief the council on Tuesday. He has said he then wants to step down and return to his job as a leading prosecutor in Berlin.
Lebanon has asked the Security Council to extend Mehlis's commission for six months after its mandate expires on Thursday. The Security Council is likely to agree to extend it until 15 June.