The top U.N. envoy on Syria says the regime of Bashar al-Assad is willing to halt the bombing of the flashpoint city of Aleppo for up to six weeks. In a briefing to the Security Council, Staffan de Mistura announced the breakthrough, while saying he has no illusions about its chance of success.

Staffan de Mistura: “The government of Syria has indicated to me its willingness to halt all aerial bombing — all aerial bombing, all sorts, all types of aerial bombing — and artillery shelling for a period of six weeks all over the city of Aleppo, from a date which we will be announcing from Damascus. Let’s be frank. I have no illusions because, based on past experiences, this will be a difficult issue to be achieved. But we will engage also very much the opposition hopefully to see them to respond to a similar request from the U.N. to halt mortar and rockets on all over the city of Aleppo for six weeks.”

Aleppo has seen intense violence in the government’s clashes with rebel groups. The Syrian civil war enters its fifth year next month.