Hamas gunmen fatally shot six bodyguards from the rival Fatah movement and mistakenly ambushed a jeep carrying their own fighters, killing five.
The streets of central Gaza City echoed with the rattle of gunfire, and were empty except for gunmen in black ski masks. Terrified Palestinian residents are hunkering down in their homes as the third ceasefire in as many days collapsed, reports CBS News correspondent Robert Berger.
Many civilians were forced to huddle together in dark homes after electricity to some downtown neighborhoods was cut off by a downed power line.
The violence threatened to bring down the Palestinians' fragile, two-month-old Hamas-Fatah unity government — and brought the Palestinians dangerously close to all-out civil war.
"What is happening in Gaza, endangers not only the unity government, but the Palestinian social fabric, the Palestinian cause and the Palestinian strategy as a whole," said Palestinian negotiator Saeb Erekat.
Despite Israel's vow to stay out of the fray, its missile strike added another layer of complexity to Gaza's mayhem, and raised the specter of a large-scale Israeli invasion.
At mid-day Wednesday, policemen from the Fatah-allied Preventive Security organization arrested five Hamas men and were driving them through Gaza City when the vehicle was ambushed by Hamas fighters, Preventive Security officials told The Associated Press. Five of the Hamas men were killed, along with two Fatah men, they said.
Hamas radio reported that a Hamas man was killed in another clash, and a nurse traveling in an ambulance was shot in the head after being caught in the crossfire, hospital officials said. Her family said she was brain dead and being kept alive by a respirator.
In four days of fighting, 41 people have been killed in the infighting, and dozens more have been injured. The majority of the fatalities have been from Fatah.
In other developments: