The U.N. 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," U.N. spokesman Stephane Dujarric said.
Ban urged "those with influence over the parties to do everything to convince them to observe the humanitarian cease-fire," he said.
"The secretary-general condemns in the strongest terms the reported violation by Hamas of the mutually agreed humanitarian ceasefire which commenced this morning," Dujarric said. "He is shocked and profoundly disappointed by these developments."
Ban noted that the U.N. 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 cease-fire came into effect.
"This would constitute a grave violation of the cease-fire, 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."
Israel and Hamas accused each other of breaking the cease-fire, which had been announced by the United States and the United Nations and took effect at 8:00 a.m. 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.
Dujarric also said Ban is also concerned at the resumption of Israeli attacks on Gaza that killed over 70 Palestinians Friday morning.
Arab nations have requested "an informal meeting" of the 193-member U.N. General Assembly to discuss "the deteriorating humanitarian situation in Gaza," according to a letter sent to Assembly President John Ashe.
U.N. political chief Jeffrey Feltman also told reporters that Ban is "profoundly disappointed" that the assurances from Hamas were not kept.
According to the Israeli army, an hour after Friday's cease-fire started, gunmen emerged from one or more Gaza tunnels and opened fire at Israeli soldier. During the ensuing mayhem, an Israeli army lieutenant was apparently captured, while another two soldiers were killed.
Feltman said he "is convinced" that Hamas knew Israel could carry out defensive operations such as tunnel destruction during the cease-fire.
"Perhaps some will deny that now, but, yes, it was very clear in the diplomacy being done yesterday ... that the Israelis were going to continue to try to do destruction of tunnels," he said. "So I don't think - rightly or wrongly - it should have been any surprise to anyone that Israelis were going to continue their tunnel operation given their own public statements."
The joint U.N.-US statement announcing Friday's cease-fire said all parties would send delegations to Cairo for negotiations aimed at reaching a lasting truce.
Feltman said the Egyptian government was hosting the talks and had issued invitations, so a decision is up to them. He said he has "a hard time envisioning how these talks would work right now" since they were predicated on a cease-fire.