“I’m seriously concerned by the escalating violence in Aleppo,” Ban said during a visit to London for the Olympic Games. “I urge the Syrian government to halt the offensive.”

“The violence from both sides must stop for the sake of suffering civilians in Syria,” he added, following a meeting with British Foreign Secretary William Hague.

Ban’s warning came as world powers voiced fears that Syrian President Bashar al-Assad may be preparing a “massacre” of his own people.

Hague earlier said the assault could spark a humanitarian disaster.

“This utterly unacceptable escalation of the conflict could lead to a devastating loss of civilian life,” Hague said in a statement.

Tanks, helicopter gunships and warplanes have poured into Aleppo over the past two days, with troops firing on a string of rebel neighbourhoods in its southwest on Friday in the battle to control the commercial hub.

Syrian rebels said Friday they had captured 100 soldiers and militia members in Aleppo.