Staffan de Mistura’s estimate, which far exceeds those given by UN in the past, is not an official number.

The UN special envoy for Syria has estimated that 400,000 people have been killed throughout the last five years of civil war, urging major and regional powers to help salvage a crumbling ceasefire.
Explaining that the death toll was based on his own estimate, Staffan de Mistura said on Friday that it was not an official UN statistic.
“We had 250,000 as a figure two years ago,” said de Mistura. “Well, two years ago was two years ago.”
The UN no longer keeps track of the death toll due to the inaccessibility of many areas and the complications of navigating conflicting statistics put forward by the Syrian government and armed opposition groups.
De Mistura also appealed to all involved parties to help revamp negotiations between the government of President Bashar al-Assad and opposition groups.
De Mistura compared the apparently stalled political talks on Syria’s future, the unravelling ceasefire agreement and the still limited humanitarian relief deliveries to the three legs of a table.
“The level of danger to the table made of three legs – and a table of three legs is always fragile by definition – [means that help] is urgently required,” he said.
“When one of them is in difficulty we can make it. When all three of them are finding some difficulty, it’s time to call the ISSG.”
He gave no date or venue for the high-level ISSG.
The envoy said he planned to continue peace talks next week, despite the “worrisome trends on the ground”, adding that he would seek clarity from government negotiators about their interpretation of political transition.

Source: Agencies, 23 Apr 2016