The United Nations' Syria envoy Staffan de Mistura is struggling to arrange a cease-fire between government and rebel forces fighting near the city of Aleppo.
De Mistura says Aleppo's population is in desperate need of food and other humanitarian aid.
He says if the city falls, it could create another 400,000 refugees.
Aleppo, Syria's second largest city, is split between government troops and rebels who are fighting to overthrow President Bashar al-Assad.
The Assad military said Sunday it killed 21 rebel fighters near Handarat Hill, north of Aleppo, where government troops are trying to seize a rebel supply line.
Official Syrian media report the military now controls the Mallah farms area near Handarat.