Palestinian officials in Gaza say Sunday was the deadliest day since the current fighting with Israel began.
More than 40 people were killed in Israeli air strikes on the territory on Sunday.
Israel’s army say Palestinian militants have fired more than 3,000 rockets at Israel over the past week.
United Nations Secretary-General António Guterres has warned that further fighting could plunge the region into an “uncontainable crisis”.
He pleaded for an immediate end to the “utterly appalling” violence.
Early on Monday, Israeli warplanes launched 80 air strikes on several areas of Gaza City, shortly after Hamas militants fired a barrage of rockets at southern Israel.
The UN has also warned of fuel shortages in Gaza which could lead to hospitals and other facilities losing power.
Lynn Hastings, UN deputy special co-ordinator for the Middle East Peace Process, told the BBC that she had appealed to Israeli authorities to allow the UN to bring in fuel and supplies but was told it was not safe.
Gaza officials said 42 people, including 16 women and 10 children, died in Sunday’s Israeli air strikes.
Ten people, including two children, have been killed in rocket attacks on Israel since the fighting began last Monday, Israel said.
The overall death toll in Gaza now stands at 188 people, including 55 children and 33 women, with 1,230 injured, according to the Hamas-controlled health ministry. Israel says dozens of militants are among the dead.
Israeli air strikes hit a busy street in Gaza just after midnight on Sunday, causing at least three buildings to collapse and dozens of deaths.
Hamas launched series of rockets towards southern Israel overnight and during the afternoon.
Millions of Israelis scrambled to safe rooms or shelters as sirens went off. Palestinians also tried to take precautions, but in the densely packed and poorly resourced Gaza Strip, many had nowhere to go.