The five Palestinian sisters were fast asleep when a night-time Israeli airstrike hit the next-door mosque in Gaza. One of the walls collapsed on to their small asbestos-roofed home and they were all killed in their beds. The eldest sister, Tahrir, was 17 years old, the youngest, Jawaher, just four.

"They grow up day after day and night after night. Within a second, I have lost them," the girls' father, Anwar Balousha, said yesterday. The 37-year-old, along with another three of his children, was himself injured in the attack on the densely populated Jabalya refugee camp.

The funerals of the sisters – Tahrir, 17; Ikram, 15; Samar, 12; Dina eight; and Jawaher, four – were attended by family members and thousands of mourners. But with space running out in the cemetery, the five girls had to be buried in just three graves, one for the eldest and the others forced to share.

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Mr Balousha wept down the phone, saying he felt "how a father who lost his five daughters would feel". With recorded readings from the Koran audible in the background, along with occasional explosions in the distance, he added: "It is the will of Allah. We are believers in God."

Amid the pile of rubble that was the Balousha home yesterday, three torn blankets could be seen poking out from the ruins along with a painted blue iron, a broken brown cupboard and a baby's bed.

The Israeli military said it had targeted the next-door mosque because it was a "known gathering place" of Hamas adherents. It said four gunmen were inside it at the time of the attack. The mosque was named Imad Akel after the former leader of the Hamas military wing.

As Israeli strikes continued, the uncle of the dead sisters said the family had been innocent victims. "We are not those who are firing rockets against Israel," Ibrahim Balousha said. "We are just people, human beings and not animals."

The Balousha family had moved out of their house when the Israeli bombing started on Saturday but they had decided to return "to meet their fate" in the words of the dead girls' uncle. He said that three missiles had been used in the airstrike at around 11.20pm on Sunday night and that hundreds of neighbours had arrived to help in the wake of the carnage.

After the funeral, 16-year-old Iman, who was briefly buried in the rubble of the family home but survived, described her unlucky siblings' dying moments. "I told my sisters, you will be martyrs, this is the end."

Her grieving uncle said that Hamas had taken advantage of the funeral to chant slogans including "Vengeance, Vengeance". Shouts of "Bomb Tel Aviv" were also heard. But Ibrahim Balousha said he had given the militant group short shrift. "I told them, this is a funeral and not a rally."

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Times were already tough for the family of refugees even before the latest tragedy. The girls' father is unemployed for 11 months of the year, picking up work selling Ka'ak bread around Ramadan. The family depend on food rations from the United Nations Relief and Works Agency and a $40 (£27.50) monthly handout. "The story is almost the same for decades," Ibrahim Balousha said: "Intifada and miseries, poverty and catastrophes."

UNRWA spokesman Christopher Gunness said: "The killing of these young girls is another tragic illustration that this bombardment is exacting a terrible price on innocent civilians. As with the killing of UNRWA students [on Saturday] we hope there will be a thorough and impartial and transparent investigation.

"Most important of all there has to be accountability. We need to know if international law was violated and if so, by whom," he added.

The UN yesterday issued a "conservative" estimate of the number of civilians killed in three days of unprecedentedly fierce aerial bombardment, putting the death toll at 62. It is a deliberately conservative estimate because it excludes all men in the Gaza City area to ensure that it does not accidentally include uniformed personnel.

The Palestinian Centre of Human Rights said that "most" of the more than 300 casualties were civilian but their tally includes Hamas policemen. It also said some bodies had still to be identified because they were so badly disfigured and that its field officers – who aim to chart every Palestinian casualty – are facing "extreme difficulties in visiting some areas, particularly those under multiple bombardment.

The Israeli military insists that it is doing its utmost to prevent civilian casualties but repeatedly points out that Hamas regularly and "cynically and specifically" uses locations in heavily built-up areas.