BEIRUT -- The United Nations' humanitarian chief says initial reports indicate some 16,000 people have been displaced following Syrian government advances on rebel-held Aleppo districts.

Stephen O'Brien says Tuesday that thousands more are likely to flee if fighting continues to spread in the coming days. He says the fleeing civilians are heading to uncertain and precarious situations amid intense attacks. O'Brien expresses concern for the civilians' fate, calling the situation in Aleppo "deeply alarming and chilling."

In swift and dramatic advances, Syrian government and allied troops pushed their way into northern parts of opposition held eastern Aleppo, touching off a wave of panic and flight from the besieged enclave.

Many of the fleeing civilians headed to government and Kurdish-controlled areas while others were driven deeper into the remaining rebel-held areas.