The U.N. humanitarian chief is urgently calling for a weekly 48-hour "humanitarian pause" in the eastern part of Aleppo city in Syria where all road access is blocked by fighting and between 250,000 and 275,000 people are trapped.

Stephen O'Brien told the Security Council on Monday that he could not stress enough "how critical the situation is" in eastern Aleppo, which risks becoming the largest besieged area in the country.

He says all sieges in the country must be lifted, a cessation of hostilities must be restored throughout Syria, and humanitarian workers must have access to help the sick and needy.

Britain's U.N. Ambassador Matthew Rycroft says he received an email Monday morning from a doctor at Aleppo Children's Hospital saying "if nothing is done we are surely facing death."