The Foreign Office is urgently investigating reports that a British national has been killed in Gaza.
The man, said to be from Rochdale, apparently died in an Israeli strike on the town of Rafah yesterday.
A spokesman for the department said: "We are aware of the reports of the death of a British national in Rafah and are urgently looking into them."
The Prime Minister said the apparent death of the manunderlined the need for "an immediate unconditional humanitarian ceasefire observed properly by both sides".
Speaking on a visit to the Loos Cemetery in northern France as part of the commemorations of the First World War centenary, 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."
The man was initially identified as working for the development agency Muslim Aid but it denied this.
Many people named him on social media but no official source confirmed his identity.
A ceasefire intended to open a "humanitarian window" in Gaza began at 8am UK time and ended at 3pm, although the Israeli military has said it will not apply to areas where troops are still operating and where they would respond to any attacks.
The ground element of the four-week operation to tackle heavy rocket fire and tunnel incursions from Gaza is believed to be scaling down, although air strikes have been continuing. An estimated 1,800 Palestinians - a third of them said to be civilians - and more than 60 Israelis have died.
At least 10 people were killed in Rafah on Sunday in an Israeli strike on a UN school that was sheltering refugees from Israel's ongoing offensive.
The US condemned the attack as "appalling" while the UN called it "criminal".
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Cameron resisted condeming Israel's behaviour, saying he was "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."
Miliband suggested that Cameron was out of step with public feeling in Britain, arguing: "The government needs to send a much clearer message to Israel that its actions in Gaza are unacceptable and unjustifiable.
"What I want to hear from David Cameron is that he believes that Israel's actions in Gaza are wrong and unjustified, and we haven't heard that from him. I think that's what the British public are thinking as they are seeing these tragic events unfolding on our television screens."
A ceasefire intended to open a "humanitarian window" in Gaza began at 8am UK time, although the Israeli military has said it will not apply to areas where troops are still operating and where they would respond to any attacks.
The ground element of the four-week operation to tackle heavy rocket fire and tunnel incursions from Gaza is believed to be scaling down, although air strikes have been continuing. An estimated 1,800 Palestinians - a third of them said to be civilians - and more than 60 Israelis have died.
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