The UN special envoy for Syria has estimated that 400,000 people have been killed throughout the past five years of civil war, urging major and regional powers to help to 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," he said. "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.

In the latest violence, at least 30 civilians were killed on Saturday in regime and rebel bombardment of areas across Syria.

Twelve people were killed in an air strike on the northern city of Aleppo, a local civil defence official said.

State news agency SANA said that three civilians were killed and 17 wounded in the rebel shelling of government-held areas of Aleppo.

The Syrian Human Rights Observatory said 13 others died in shelling of the rebel-held town of Douma, east of Damascus, while two men were killed in regime air strikes on Talbiseh in central Homs province.

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De Mistura also appealed to all involved parties to help revamp negotiations between the government of President Bashar al-Assad and opposition groups.

"Yes we certainly do need a new ISSG at ministerial level," the envoy said, referring to the International Syria Support Group which includes the United States, Russia, the EU, Iran, Turkey and Arab states.

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.

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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.

The government, which says the future of President Bashar al-Assad is not up for discussion in Geneva, says that political transition will come in the shape of a national unity government including current officials, opposition and independent figures.

"Is this going to be cosmetic, is this going to be real, and if it is real what does it mean for the opposition and so on?" he said.

Opposition negotiators have rejected any proposal which leaves Assad in power. They have also accused the government of violating a February "cessation of hostilities" agreement, pointing to air strikes on rebel-held areas which have killed dozens of people this week.