"I strongly reiterate the appeal of the (UN) secretary-general to all concerned in Syria and the region and beyond to de-escalate immediately and unconditionally, and urge all stakeholders ... to use their influence to help reduce violence."
There has been a gradual return to back-and-forth competition over territory in Idlib and Hama provinces, and heavy and sustained airstrikes across the northwest and in besieged eastern Ghouta, said de Mistura.
Civilians has been killed on a horrific scale and strikes have continued to hit hospitals, schools and markets.
Reports suggest that more than 1,000 civilians lost their lives in the first week of February alone, he said.
There have been several allegations of chemical weapons attacks in Ghouta and Idlib, and in the Afrin district on the border with Turkey, he added.
Although those allegations cannot be verified as there is no independent investigation, they are extremely worrisome, said de Mistura.
At least 320,000 people have been displaced due to fighting in Idlib in just two months, an area that is already hosting over 1.2 million internally displaced people, he added.
The conflict is also further spilling over Syria's borders in more than one direction and there have been increased military intervention from multiple sources, de Mistura noted.
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