The US has said the shelling of a UN shelter in Gaza is "totally unacceptable and totally indefensible".
In its strongest criticism yet of Israel's offensive in the Palestinian territory, the US - Israel's closest ally - urged Israel to do more to protect civilian life.
A quarter of Gaza's population has been displaced by the fighting, the UN says.
Israel says its operation in Gaza is designed to defend its population from attacks by Palestinian militants.
It blames the Hamas militant group for most of the civilian deaths in Gaza, saying its fighters deliberately operate from civilian areas.
A senior Israeli official has told the BBC that the army has now "neutralised" 70-80% of Hamas' offensive tunnel network into Israel.
Israel says it will not stop its operation in Gaza until all the tunnels - which militants use to infiltrate Israeli territory - have been destroyed.
Since Israel began its offensive in Gaza on 8 July, 1,422 Palestinians have been killed and 8,265 injured, most of them civilians, according to Gaza's health ministry.
This means more Palestinians have now been killed than during Operation Cast Lead - the last time Israel launched a ground invasion in Gaza - in 2008-09.
Fifty-eight Israelis have been killed in total - 56 soldiers and two civilians. A Thai worker in Israel has also died.
"There is a difference in approach between what Hamas is perpetrating on the Israeli people and what Israel is doing to defend their country," White House spokesman Josh Earnest told reporters.
"But the shelling of a UN facility that is housing innocent civilians who are fleeing violence is totally unacceptable and totally indefensible, and it is clear that we need our allies in Israel to do more to live up to high standards that they have set for themselves."
He was referring to an incident on Wednesday, when at least 16 people were killed when shellfire hit a UN-run school designated as a civilian shelter in the Jabaliya district of Gaza City.
Mr Earnest said there was little doubt that the shells were fired by the Israeli military.
In 2012, Israel received $3.1bn (£1.8bn) of US aid, the vast majority of it military aid.
Israeli government spokesman Mark Regev told the BBC earlier that Israel would apologise if it discovered it was responsible for the shelling of the shelter.
"We have a policy - we don't target civilians," he said.
The BBC's Martin Patience at the scene in Gaza:
In a sweltering apartment, 60 members of an extended family have sought shelter.
It is a miserable scene. They have only one hour of electricity a day, no running water or food in the fridge.
Even amid the darkness of war, the children still play games inside the apartment.
Children five or older here have already lived through three conflicts during their short lives.
One of the men told me that they had been ordered out of their neighbourhood by Israel. He said his home had been completely destroyed.
The UN says up to a quarter of Gaza's population has been displaced by the fighting.
More than 200,000 Palestinians are sheltering at UN schools. But the UN believes a similar number are crammed into homes of their relatives and friends across the territory.
The BBC's Bethany Bell at the scene in Jerusalem:
This is the longest conflict between Israel and militants in Gaza, but public backing in Israel for the army's offensive remains strong.
That could start to change if the number of Israeli casualties increases significantly, or if there are kidnappings of Israeli soldiers in Gaza.
But for now many Israelis support Mr Netanyahu's stated aim of destroying the cross-border tunnels.
The threat of Palestinian gunmen coming up from below the ground into Israeli homes and communities to kill and abduct has shaken people here.
Some Israelis are beginning to ask if the government and the army underestimated the threat of the tunnels, but there is little debate about the mission to destroy them.
The UN has also condemned the continuing violence.
"The reality of Gaza today is that no place is safe," UN humanitarian chief Baroness Valerie Amos told the Security Council.
The head of Unrwa, the main UN relief agency in Gaza, warned that "the population is facing a precipice".
"Should further large-scale displacement indeed occur, the occupying power [Israel], according to international humanitarian law, will have to assume direct responsibility to assist these people," Pierre Kraehenbuehl said.
Focus on tunnels
Israel's offensive began with a focus on Islamist militant group Hamas' rocket-launching capability, but it has since expanded to take in the threat from tunnels.
The Israel Defense Forces (IDF) recently discovered an extensive network of tunnels leading from Gaza into Israel, and on the night of 17 July launched an operation to destroy them.
Reports from Israel suggest the discovery of the tunnels - and the reality that infiltrators have used them to kill Israeli soldiers inside their own country - has shocked many Israelis and bolstered support for the operation.
Israeli Prime Minister Benjamin Netanyahu said he would not accept any ceasefire that did not allow troops to continue destroying the tunnels.
Hamas says it will not stop fighting until a blockade, maintained by both Israel and Egypt, is lifted.
Israel occupied Gaza in the 1967 Middle East war and only pulled its troops and settlers out in 2005.
Israel considered this the end of the occupation, but it still exercises control over most of Gaza's borders, water and airspace. Egypt controls Gaza's southern border.