The UN inquiry into the assassination of former Lebanese PM Rafik Hariri has made progress and has found Syria more co-operative, its latest report says.
Mr Brammertz took over as head of the UN probe in January
But the chief UN investigator, Serge Brammertz, said his inquiry was in a particularly delicate stage and it could not yet name Mr Hariri's killers.
Previous reports implicated Syrian and Lebanese officials in the killing.
Damascus has denied the charges but was forced to pull its forces out of Lebanon in the wake of the death.
In December, the Security Council voted to extend the inquiry's mandate by six months.
Mr Brammertz's report to the council on Tuesday was his first since he replaced Detlev Mehlis in January.
In the 25-page report, Mr Brammertz said the UN commission had made progress in identifying how the killing was carried out.
"The commission is closer to a more complete understanding of how the preparatory work was undertaken, how those who participated on the day performed their respective tasks, what those tasks were before, during and after the attack and of the overall modus operandi employed by the perpetrators for the attack," the report said.
The report said the killing had been a highly complex "terrorist operation" carried out by people who were "very 'professional' in their approach, as they planned to a high percentage likelihood for success, and conducted the operation with high standards of individual and collective self-discipline.
"It must be assumed that at least some of those involved were likely experienced in this type of terrorist activity," it added.
The report also said co-operation from the Syrian government had increased since Mr Brammertz took over the inquiry from Mr Mehlis, who had called Damascus obstructive.
A UN Security Council resolution passed in December threatened further action unless Syria co-operated fully with the probe.
"The Syrian government has, in particular in the last three months, formally complied with nearly all of the Commission's previous requests for assistance," it said.
Damascus had provided a number of responses on specific issues raised by the inquiry, the report said.
"Despite these encouraging steps, it is important to note that the Commission will ultimately judge co-operation of the Syrian authorities on the merits of the information provided and the promptness with which its requests are being accommodated," it added.
The report said investigators would meet Syrian President Bashar al-Assad and Vice-President Farouk al-Sharaa in April.
Mr Assad has refused earlier requests to meet with the commission.