David Cameron has refused to say whether he believes Israel's actions in Gaza are "criminal" after a brief ceasefire brought a lull in the bloodshed.
The Prime Minister said the UN was "right" to condemn an airstrike near a school in Rafah that left 10 people dead. But he stopped short of endorsing secretary general Ban Ki-Moon's description of the attack as a "moral outrage and a criminal act".
The comments came amid reports that a British aid worker from Rochdale was among those killed in the military campaign yesterday.
Tel Aviv declared a "humanitarian window" in designated parts of Gaza from 7am UK time until this evening.
Palestinian officials accused Israel of breaking the truce within minutes by bombing a house in Gaza City, while a Palestinian construction worker was apparently shot dead after an attack on a bus in Jerusalem that claimed the life of an Israeli civilian.
But reports indicated a significant reduction in violence.
The ground element of the four-week operation to tackle heavy rocket fire and tunnel incursions from Gaza is believed to be scaling down, but the air campaign shows little signs of ending. An estimated 1,800 Palestinians - a third of them said to be civilians - and more than 60 Israelis have died.
Ten people were killed in the latest incident at a UN school in the southern Gaza town of Rafah, which had been converted into a shelter. The Red Crescent charity said the attack happened while people were queuing to get food from aid workers.
The Prime Minister was criticised by Ed Miliband over the weekend for failing to take a tough enough line with Tel Aviv over the crisis.
The Labour leader said the UK needed to send a clear message that the military operation was "wrong and unacceptable".
Speaking on BBC Breakfast this morning, Mr Cameron said: "I think the UN is right to speak out in the way that it has, because international law is very clear that there must not be the targeting of civilians or the targeting of schools, if that is what has happened.
"The UN, as the international body, is right to speak out in the way it has."
Pressed on whether he regarded the attack as a "criminal act", Mr Cameron replied: "The UN has spoken very clearly and I think they are right to speak very clearly...
"I'm not an international lawyer, so that's up to international lawyers. But international law is very, very clear that the use of force always has to be proportionate and civilians should not be targeted."
Mr Cameron said the Government had been "very clear that there needs to be an immediate comprehensive humanitarian ceasefire, that we want this conflict to stop".
"We obviously do think it is appalling, the loss of life that there has been," he said. "From the start, though, we have also made the point that if the Hamas rocket attacks on Israel stop, then that would be probably the fastest way to stop this conflict."
Mr Cameron said the Government was urgently investigating reports that an aid worker from Rochdale had been killed in an air strike last night.
Speaking on a visit to the Loos Cemetery in northern France, Mr Cameron said: "I'm extremely concerned about these reports and we are doing everything we can to get to the bottom (of them) and find out exactly what has happened.
"I don't want to say anything before we've been able to do that but this only reinforces the need for an immediate unconditional humanitarian ceasefire, observed properly by both sides.
"This slaughter, this killing has got to end."
Downing Street confirmed that all licences for exporting arms to Israel were being reviewed, although no decisions have been taken on whether to withdraw any.
International Development Secretary Justine Greening announced that the UK is to provide an extra £2 million in emergency assistance to help provide mattresses, blankets, nappies, cooking equipment and other essential supplies for nearly 8,000 families who have fled the conflict in Gaza.
The cash will be contributed to the United Nations Relief and Works Agency's (UNRWA) Gaza flash appeal and brings the UK's aid funding for Gaza up to £15 million in the last two weeks.
Israel's ambassador to London Daniel Taub told BBC Radio 4's Today programme that the human suffering and loss of life in Gaza was "appalling".
But he added: "I think we need to stop for a moment and remember exactly what is happening.
"Hamas's strategy... is to operate from the vicinity of UN facilities, of hospitals.
"We are aware that there are terrorist organisations across the world... who are watching what is happening in Gaza asking themselves 'Have we found the Achilles heel of democracies fighting terrorism?'.
"In that situation, not responding is simply not an option."
Asked whether Israel would still strike if its own citizens were the victims, he replied: "The answer is yes. We cannot have a situation where terrorists prove that hiding behind civilians is a way of getting carte blanche."
The ambassador called the most recent attack on a UN school a "tragic incident" and stressed that Israel was investigating what had happened.
But he added: "Three times over the past week and a half we have found UN schools... that have been used as stockpiles, as storage houses. It is clearly not appropriate that schools, hospitals should be used for this sort of activity."
Shadow foreign secretary Douglas Alexander welcomed the temporary ceasefire today but warned that the killing of civilians and children in Gaza in recent days represented "both a moral failure and a strategic error".
He said: "Today's latest news, both of a reported terror attack in Jerusalem and reports of the tragic death of a British aid worker in Gaza, only confirms the suffering that continues.
"Hamas displays no regard for human life and must cease firing rockets into Israel and digging tunnels to facilitate the murder of civilians.
"But sustainable security for Israel cannot be achieved simply by permanent blockade, aerial bombardment and periodic ground incursion.
"Instead, it requires acknowledging that Palestinian statehood is not a gift to be given but a right to be recognised, and sustained efforts must be made to move beyond a profoundly unjust status quo."
Labour MP Ann Clwyd, who chairs the all-p arty parliamentary group on human rights, told Foreign Secretary Philip Hammond that the UK and others need to be clearer in telling the Israeli government that its actions we re "legally and morally wrong" and "counter-productive".
In a letter to Mr Hammond, she questioned what the UK proposes to do on its own and with its allies to protect civilians and end the conflict in Gaza.
Ms Clwyd said Hamas must stop firing rockets into Israel but added despite Israel's right of self-defence, it was wrong to respond by carrying out its current operation on Gaza.
The MP for Cynon Valley questioned why the UK had been reticent in condemning Israel, adding to Mr Hammond in her letter that Israel's actions has led to some Palestinians becoming "more extremist".
Ms Clwyd told the Foreign Secretary : " You would no doubt agree that while weakening Hamas' military capability and destroying the tunnels in Gaza may be viewed by Israel as a significant victory now, Israel's actions can only serve to inflame many Palestinians, with some becoming more extremist.
"Enforced isolation has already resulted in half of Gazans living in poverty. These attacks will generate further despair, dislocation and deprivation - with the killing and maiming of relatives, widespread displacement and destruction of infrastructure.
"The UN has warned of a 'health disaster of widespread proportions', with medical services facing collapse.
"As you have acknowledged, an immediate and unconditional ceasefire is urgently needed. That is the first step. The international community must then get all sides to negotiate a just settlement to stop this cycle of violence.
"The Middle East peace process can no longer be mainly the US's responsibility but that of all wishing to see peace in two states, Israel and Palestine, as well as across the Middle East. What will the UK be doing in that regard?"
Conservative former minister Crispin Blunt said both sides in the conflict were "arguably committing war crimes" while the Government should retain the option to impose sanctions on Israel.
Speaking to Channel 4 News, Mr Blunt said: " You've got both sides arguably committing war crimes here, which is why the slaughter is so appalling in Gaza and we've got to have a route to solve the overall situation, which means a route to a settlement of the Middle East peace issue between Israel and the Palestinians."
He added: "It's fairly straight-forward that firing rockets indiscriminately at people is against the Geneva Convention, as is the level of disproportionate responses coming back from Israel and this is the third time Israel has had to run a very serious operation into Gaza of this kind.
"Now, over the whole of that period you can make a case, that I think stands up, that Israel has actually obstructed rather than enabled a route to a settlement of the overall conflict. It complains it does not have a negotiating partner - well, it ought to get serious about helping its partner."
Mr Blunt said Israel should be told it is expected to adhere to the highest possible standards and to be serious to make "very difficult concessions" required to reach a solution.
Asked if the UK Government should look at sanctions against Israel, Mr Blunt said: "Of course it has to have that in its pocket. It's in everyone's interest that we address this issue properly.
"We've got to understand we've got two sides here, both of which enjoy popular support from their people for conflict which in the end is just leading us absolutely nowhere but to the murder and mutilation of hundreds if not thousands of innocent people."