The Syrian government has agreed to suspend airstrikes and artillery shelling on Aleppo for six weeks to see if a ceasefire is workable in the city, according to the United Nations envoy to Syria.

Staffan de Mistura was briefing the Security Council in closed session on his latest efforts to find a solution to the civil war. He called the new development a "glimmer of hope" and continued to emphasise a political solution to the conflict.

But it remains unclear when the trial ceasefire would take place.

Mr de Mistura now has to get the opposition's support for the plan, which includes a request for them to suspend rocket and mortar fire in the same period. Syria's ambassador to the UN refused to comment after the council meeting.

"Let's be frank. I have no illusions," Mr de Mistura told reporters. "Based on past experiences, it is a difficult issue to achieve."

This was Mr de Mistura's first council briefing since he explained his freeze plan in October, and council members wanted to know what kind of support, it received from president Bashar Assad.

Aleppo is divided into a rebel-controlled west and government-held east. Mr de Mistura wants to see a UN-monitored "freeze zone" that will calm violence there, allow more humanitarian aid access and act as the first step toward a wider solution to the conflict.

"Our hope is that Aleppo could be a signal of goodwill, a confidence-building measure which could and can facilitate the re-starting of a political process with a clear political horizon," he said last month in Geneva.

But Aleppo-based opposition activists have expressed fears the government would exploit a truce to gather its forces to fight elsewhere, and they have questioned how a ceasefire could work with Islamic State fighters in the area.

The UN representative of the opposition Syrian National Coalition, Najib Ghadbian, says they are waiting to see a detailed proposal from Mr de Mistura.

"The Assad regime's compliance with any such proposal will be judged by actions, not words. And thus far, his actions have been only brutality and terror," he said.

The UN estimates that the conflict has killed 220,000 people. Millions have fled to neighbouring countries.