Russian Foreign Minister Sergey Lavrov(R) attends a high-level meeting on Syria at the Security Council Chamber at the UN Headquarters in New York, Sept 21, 2016. [Photo/Xinhua]

Global norms of humanitarian law have been "flagrantly violated," defying the Security Council's resolutions, he said.

De Mistura has been struggling to keep the stalled peace process alive amid a surge in fighting between Syrian forces and rebel fighters.

At the same time, the secretary-general said that it is known that international unity can make a difference as council unity and resolve has led to the elimination of Syria's previously denied chemical weapons programme and the attribution of responsibility for their use.

Moreover, food and medicines have been delivered to millions of Syrians, also across front lines and via air drops to besieged and hard-to-reach areas. The formation of the International Syria Support Group has also provided fresh momentum to the search for a settlement and paved the way for Security Council resolution 2254, adopted in 2015, the UN chief said.

Meanwhile, Ban also recalled that the cessation of hostilities -- albeit fragile -- that began in late February yielded positive results for a few months.

In that window, the UN intensified humanitarian operations and brought the parties to Geneva for talks, he said.

"But that process was once again overwhelmed by violence," the secretary-general said, noting that the "long-sought" agreement between the Russian Federation and the United States, concluded on Sept 9 "represents a new opportunity."