The death toll in Gaza has topped 1,400, with more than 40 people dying after another day of intense Israeli bombardment from air, sea and land.

The toll is now greater than in both previous rounds of fighting between Israel and Hamas. Israeli military losses are also significantly higher.

Palestinian officials in Gaza said on Thursday that 8,200 people had been wounded in the four-week operation. Up to 80% of the Palestinian casualties were civilians, according to local non-government organisations and the UN.

Three civilians on the Israeli side and 56 soldiers have been killed so far.

In Gaza City, Abu Ahmed, 65, said the situation was the worst he had ever known. "I have experienced everything – the 1967 war, two intifada [uprisings]. By chance we are alive. But we don't know if we die now, today or tomorrow," the shopkeeper said.

Much of Gaza receives less than two hours of electricity a day, while medicine and safe water are increasingly scarce. Officials fear the development of health epidemics as sanitation systems break down. Some basic foodstuffs in Gaza City, such as tomatoes, now cost five times more than three weeks ago.

Valerie Amos, the UN's emergency relief coordinator, told the security council the world had watched "in horror the desperation of children and civilians that have come under attack". Philippe Krahenbul, the most senior official representing the UN Relief and Works Agency for Palestinian refugees, said Gaza was "facing a precipice" and "the illegal blockade of Gaza must be lifted".

The Israeli offensive, Operation Protective Edge, was launched with the stated aim of ending cross-border rocket fire from Gaza as well as destroying tunnels used by Palestinian militants to infiltrate Israel. During Operation Cast Lead in 2008-09, up to 1,400 people were killed in Gaza, according to Palestinian rights groups. In 2012's Operation Pillar of Defence, 133 Palestinians were killed.

Rockets continued to be fired from Gaza into Israel on Thursday, many originating from densely populated urban areas. Two people were wounded by rockets in the southern town of Kiryat Gat, Israeli media reported. Another eight Israelis were injured by mortar fire near the Gaza border, Haaretz reported. More than 2,800 rockets have been fired into Israel in recent weeks, according to officials. Most have been intercepted by the "Iron Dome" missile defence system.

Amos detailed attacks on more than 103 UN facilities, including one on a school on Wednesday that killed 19 people and injured more than 100.

The White House increased its public pressure on Israel to avoid further civilian deaths after concluding that its forces were likely to have been behind the shelling of a UN school full of refugees.

"The shelling of a UN facility, that is housing innocent civilians who are fleeing violence, is totally unacceptable and totally indefensible," Barack Obama's spokesman said. "It is clear that we need our allies in Israel to do more to live up to the high standards they have set themselves."

Israeli officials have said its forces were trying to avoid civilian casualties and blamed these on Hamas and other Palestinian factions who it said were embedded in urban areas. Israel said its forces were attacked by guerrillas near the UN school in northern Jabaliya and had fired back. In another incident on Wednesday, 17 people were killed in nearby Shujai'ya by what Palestinian officials said was Israeli shelling of a produce market. The Israeli military said it was investigating.

In central Gaza City on Thursday, three explosions destroyed the home of the Ramlawi family, where 50 refugees were crammed into its three storeys.

Rolling Israeli ground assaults on residential areas, preceded by mass warnings to evacuate, have displaced more than 200,000 of Gaza's 1.8 million Palestinians. Any accommodation away from the areas hit hardest by the shelling and bombing is packed with refugees.

An officer from the Israel Defence Forces phoned Mahmud Ramlawi at about 2pm to warn him of the impending strike. The 20-year-old then received five successive calls from the officer, who called himself Musa, asking if the family had evacuated the premises. "We need more time," Ramlawi said as his brothers searched for valuables and documents."He was very cool, very calm. I argued with him, I asked: 'Why destroy my house?' He said he would call me afterwards and explain," the civil servant recounted.

A warning missile was fired at the house, followed by a final call came from the officer and three successive blasts which almost levelled the home Ramlawi's father had built over 20 years.

"Why have they destroyed our house?" asked Meher Ramlawi, 35, Mahmud's brother. "Were they scared of all our wives and daughters and sisters who live there?"

Meher, whose seven-year-old daughter is in a critical condition after being caught in shellfire earlier this week, said Palestinians and Israelis could not live together. "This is a war. One side has to finish the other."

Moshe Yaalon, Israel's defence minister, said on Thursday that progress towards military goals had been satisfactory: "We are completing our treatment of the terror tunnels.During the fighting, soldiers are finding new tunnel shafts, and they are also being neutralised."

Meher Ramlawi, whose daughter, 7, is in a critical condition after being caught in shellfire earlier this week, said Palestinians and Israelis "cannot live together" "This is a war. One side has to finish the other," he said.