The U.N.'s special envoy for Syria is briefing the Security Council on Tuesday after angering Syrian opposition leaders last week with his comments that President Bashar Assad remains part of the solution in reducing violence in the nearly four-year conflict.

Staffan de Mistura will speak to the council behind closed doors on his meeting last week with Assad about the envoy's proposal to freeze hostilities in the northern city of Aleppo.

This is de Mistura's first council briefing since he explained the plan in October.

Opposition groups question whether his idea is going anywhere. The U.N. representative of the opposition Syrian National Coalition last week told reporters that "if this is not working, we should not waste time," instead emphasizing a political solution.

The U.N. estimates the conflict has killed 220,000 people.