The U.N. says its special envoy to Syria is heading to Damascus on Saturday to meet with senior government officials on key details of a proposed six-week suspension of government airstrikes and artillery shelling in the country's largest city, Aleppo.

U.N. Security Council members on Thursday expressed support for Staffan de Mistura's efforts as the Syrian conflict is set to enter its fifth year next month.

Even Syria ally Russia urged the Syrian government to "go the extra mile" to find an agreement with de Mistura on the plan, which would allow the U.N. to test the unrestricted delivery of badly needed humanitarian aid to one district of the devastated city.

But U.S. Ambassador Samantha Power warned that Syria's government "has an abysmal track record in honoring its commitments."