More than 200,000 Syrians have fled the city of Aleppo amid intense clashes between regime forces bombarding rebels who have seized large swathes of the city.
Valerie Amos, the top UN official for humanitarian affairs, said: "I am extremely concerned by the impact of shelling and use of tanks and other heavy weapons on people.
"Many people have sought temporary shelter in schools and other public buildings in safer areas," she added. "They urgently need food, mattresses and blankets, hygiene supplies and drinking water."
Ms Amos said UN agencies and the Syrian Red Crescent were working together on supplying those affected by the fighting all over the country with blankets and humanitarian supplies, but many remain out of their reach because of the combat.
"It is not known how many people remain trapped in places where fighting continues today," she warned. Aleppo is Syria's largest city and commercial hub with about three million inhabitants.
Fleeing residents have described incessant shelling, shortages of food and petrol and soaring black market prices for everyday staples.
US Defence Secretary Leon Panetta said that the use of heavy weapons, particularly helicopters, is just another nail in President Bashar Assad's coffin.