The UN chief urged both sides "to show maximum restraint and return to the agreed 72-hour humanitarian cease-fire that tragically lasted such a brief period of time".
Israel and Hamas are accusing each other of breaking the ceasefire, which had been announced by the United States and the United Nations and took effect at 8am local time on Friday. The fighting broke out less than two hours later, and the apparent capture of the Israeli soldier could lead to a major escalation of the 25-day-old conflict.
"The secretary-general condemns in the strongest terms the reported violation by Hamas of the mutually agreed humanitarian ceasefire which commenced this morning," UN spokesman Stephane Dujarric said. "He is shocked and profoundly disappointed by these developments."
Ban noted that the UN has no independent means to verify exactly what happened Friday morning, Dujarric said, but according to the latest reports, two IsraeIi soldiers were killed and one taken captive after the ceasefire came into effect.
"This would constitute a grave violation of the ceasefire, and one that is likely to have very serious consequences for the people of Gaza, Israel and beyond," Dujarric said. "Such moves call into question the credibility of Hamas' assurances to the United Nations."
The UN chief is also deeply concerned at the resumption of Israeli attacks on Gaza.
"Instead of giving both sides, especially Gazan civilians, a much needed reprieve to let them attend to their injured, bury their dead and repair vital infrastructure, this breach of the ceasefire is now leading to a renewed escalation," Dujarric said.
UN political chief Jeffrey Feltman told reporters that Ban is "profoundly disappointed" that the assurances from Hamas were not kept.
"Our goal was very clear: we need to end the killing," Feltman said.
He said the capture of the Israeli soldier will make it more difficult to get back to the point when a ceasefire was announced.
Dujarric said the secretary-general is urging "those with influence over the parties to do everything to convince them to observe the humanitarian ceasefire".