The United Nations said Sunday that 200,000 people have fled the Syrian city of Aleppo in two days as President Bashar al-Assad's forces step up their assault.
UN humanitarian chief Valerie Amos said in a statement that an unknown number of people are trapped in the city and appealed for safe access to Aleppo for aid groups.
The UN under secretary general said the International Committee of the Red Cross and the Syrian Arab Red Crescent had estimated that 200,000 people have fled Aleppo and surrounding areas in the last two days.
"It is not known how many people remain trapped in places where fighting continues today," Amos said. Aleppo normally has a population of about 2.5 million people.
Amos said that she was "extremely concerned by the impact of shelling and use of tanks and other heavy weapons" on civilians in Aleppo, Damascus and other locations.
Tens of thousands of Syrians have sought to cross into neighboring Jordan, Iraq and Turkey in recent days.
Amos said that many people in Aleppo had sought shelter in schools and other public buildings. "They urgently need food, mattresses and blankets, hygiene supplies and drinking water," she said.
"I call on all parties to the fighting to ensure that they do not target civilians and that they allow humanitarian organizations safe access" to deliver aid.
"The security situation in cities and along main transport routes is making it very difficult for humanitarian agencies to reach displaced families in Aleppo, Hama and other areas," Amos said.
Despite the "very dangerous" conditions, the Red Crescent and UN and private agencies were still delivering food, blankets and hygiene kits whenever possible, she added.
Syrian activists say more than 20,000 people have been killed in the conflict in the past 16 months.