United Nations envoy Staffan de Mistura said Friday the Syrian people, not foreigners, should decide the fate of their President Bashar al-Assad.

"Can't we leave the Syrians to actually decide on that? Why should we be saying in advance what the Syrians should say, as long as they have the freedom and the opportunity of saying so?" de Mistura told France 24 TV.

"We say that it is supposed to be a solution Syrian-led, Syrian-owned," he added.

De Mistura said he remained optimistic about the ceasefire that came into force last weekend, despite continued breaches.

"Yesterday, there were four people killed. Very sad," he said.

"But do you know how many were dying just two weeks ago? Up to 120 per day. An average of between 60 and 80 every day.

"How many people were being reached by humanitarian aid in the 18 besieged areas? Zero. What happened in the last 10 days? Well, 242 truck-loads of humanitarian aid reaching seven of those areas."

He said 115,000 people had now received aid, but accepted this was still "not enough".

He refused to comment on the impact of Russian air strikes in support of Syrian President Bashar al-Assad.

"I'm a mediator, I am a UN official, so I will not make a judgement. I think history will judge all of this.

"There is certainly a case in saying that once the Russians got involved militarily, I want to believe that they also felt… that when you get involved you (have to) become part of the solution."