Israeli forces and militants from Gaza resumed cross-border attacks after a three-day cease-fire ended at 8 a.m. local time.
Talks mediated by Egypt failed to extend the 72-hour truce.
The Israeli military launched air strikes after rockets were fired into southern Israel as the cease-fire ended.
Palestinian officials said that a 10-year-old boy had been killed and several people wounded by an Israeli air strike in northern Gaza City.
Local residents returned to UN shelters to seek refuge.
At a UN school, Imada al-Attar said: “I feel safe here, I’m afraid to go back home. However, the Israelis have been shelling places near the school and the shrapnel has reached the school.”
In Gaza City, worshippers gathered for Friday prayers at the ruins of a mosque recently hit by an air strike.