The United Nations says hundreds of thousands of civilians could be cut off from food if Syrian government forces encircle rebel-held parts of Aleppo.
Syrian government forces, backed by Russian air strikes and Iranian and Lebanese Hezbollah fighters, have launched a major offensive in the countryside around Aleppo, which has been divided between government and rebel control for years.
The assault to surround Aleppo, once Syria's biggest city with two million people, amounts to one of the most important shifts of momentum in the five-year civil war that has killed 250,000 people and already driven 11 million from their homes.
Since last week, fighting has already wrecked the first attempt at peace talks for two years and led rebel fighters to speak about losing their northern power base altogether.
The UN is worried the government advance could cut off the last link for civilians in rebel-held parts of Aleppo with the main Turkish border crossing, which has long served as the lifeline for insurgent-controlled territory.
"It would leave up to 300,000 people, still residing in the city, cut off from humanitarian aid unless cross-line access could be negotiated," the UN Office for Coordination of Humanitarian Affairs said.
It also said that if government advances around the city continue, "local councils in the city estimate that some 100,000 - 150,000 civilians may flee".
Turkey, already home to 2.5 million Syrians, the world's biggest refugee population, has so far kept its frontier mostly closed to the latest wave of displaced, making it more difficult to reach them with urgently needed aid.
The United Nations urged Ankara on Tuesday to open the border and has called on other countries to assist Turkey with aid.
Foreign Minister Mevlut Cavusoglu said as many as a million refugees could arrive if the Russian-Syrian campaign continues.