"I'm seriously concerned by the escalating violence in Aleppo," Ban said on Friday during a visit to London for the Olympic Games.
"I urge the Syrian government to halt the offensive.
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"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 on Friday they had captured 100 soldiers and militia members in Aleppo, according to a video distributed by Syrian activists.
The video released on Friday shows dozens of men in civilian clothing seated in a courtyard, and was filmed by a man who describes himself as a member of the rebel Free Syrian Army.
The captured men tell the camera that they are members of the Syrian army and the "shabiha" pro-regime militia.
"I am Colonel Mohammed Rashid Mohammed al-Mhmeid ... I was arrested at the entrance of Aleppo," one of the men says.
"I am Captain Fuad al-Hosh, commander of the Aleppo police in Shaar district. I was captured when I left my post," another said.
One man identified himself as a member of the shabiha, saying he was captured in Hanano in Aleppo and had "killed a demonstrator".
