BEIRUT (Reuters) - Up to 16,000 people have been displaced in Syria’s Aleppo by intense attacks on the rebel-held eastern part of the city, the United Nations humanitarian chief and relief coordinator Stephen O’Brien said on Tuesday, citing initial reports.

The area had no functioning hospitals left, food stocks were nearly exhausted and it was likely that thousands more people would flee their homes if fighting persisted in the coming days, he said in an emailed statement.

“The situation is very bad. There’s intense fear of collective annihilation,” said a medic who lives in the area and gave his name as Abu al-Abbas.

“This week I’ve changed locations three times,” he added, speaking on Monday using a social networking site. “In the shelter, we had dead people who we couldn’t take out because the bombardment was so intense.”

The Syrian army and its allies made a sweeping advance across the northern part of besieged eastern Aleppo on Sunday night and Monday as rebels pulled back to a more defensible front line after losing control of a key district.

Aleppo has become the most pressing battle in Syria’s war, pitting President Bashar al-Assad, backed by Russia, Iran and Shi’ite militias, against mostly Sunni rebel groups including some supported by the United States, Turkey and Gulf monarchies.

Conditions in its rebel-held eastern districts were already difficult after the army and its allies managed to impose a siege over the summer, followed by heavy bombardments using artillery, warplanes and helicopters dropping barrel bombs.

However, the fighting has escalated after the army began a new offensive last week, bringing more eastern Aleppo districts close to the front line as rescue and ambulance workers say their vehicles and equipment are running out of fuel.

The Syrian Observatory for Human Rights, a Britain-based war monitor, said warplanes pounded eastern Aleppo districts overnight, killing at least 18 people, including 12 in al-Shaar district near the new front line.

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Syrian state news agency SANA said on Monday that rebel shelling had killed seven people in government-held districts of the city.