Israeli air strike believed to be aimed at a Hamas official kills four children from one family in Gaza City ITN

At least 11 members of one family, including five women and four children, were killed when Israel bombed a house in Gaza City on Sunday as the five-day-old war claimed more civilian lives with no sign of a let-up in the intense bombardment.

The air strike flattened the home of the Dalou family in the Sheikh Radwan district of Gaza City, causing the biggest death toll in a single incident since the offensive began last Wednesday.

On Monday morning the Israeli defence force appeared to admit the family had been killed by mistake. The Haaretz website quoted the army as saying their house was either incorrectly pinpointed or a missile malfunctioned.

Elsewhere in the city early Monday an air strike levelled two houses belonging to a single family, killing two children and two adults and injuring 42 people, including children, said Gaza heath official Ashraf al-Kidra. Rescue workers were frantically searching for 12 to 15 members of the Azzam family under the rubble. Shortly afterwards, Israeli aircraft bombarded the remains of the former national security compound in Gaza City. Al-Kidra said flying shrapnel killed one child and wounded others living nearby.

After the Dalou family home was destroyed, the bodies of the children were pulled from the rubble and taken to the morgue at Shifa hospital. The dead also included an 80-year-old woman.

Ismael Haniyeh, the Hamas prime minister, described the deaths as an "ugly massacre" and the Hamas military wing, al-Qassam Brigades, said: "The massacre of the Dalou family will not pass without punishment."

Diggers at the scene of the explosion were scooping rubble from flattened buildings as rescuers tried to locate survivors.

Witnesses said there were chaotic scenes as the dead and injured were brought to the Shifa hospital, which has been on emergency footing since the start of Operation Pillar of Defence.

The bodies of four young children lay on two metal trays in the morgue, covered in dust and blood. A crowd of onlookers outside became increasingly distressed as the body of the children's mother was wheeled in, covered in blankets.

The strike was originally thought to have been aimed at a Hamas official, Mohamed Dalou.

In all, 84 Palestinians, about half of them civilians, have been killed in the five-day onslaught and 720 have been wounded. Three Israeli civilians have died from Palestinian rocket fire and dozens have been wounded.

After an overnight lull in rocket fire from Gaza, Hamas and other groups renewed their attacks on Sunday. Sirens were activated in Tel Aviv for the third day running. Israel Army Radio reported that two rockets fired at the city were shot down by Iron Dome defence system. Late night, there were reports of renewed shelling in Gaza by Israeli navel vessels.

The tactic of targeting militants' homes carries the risk of further high casualties in the densely populated areas of Gaza's main cities. Many militants have moved their families to safer areas before going into hiding themselves. The Israeli military has also expanded the scope of its targets to take in Hamas-run government offices and compounds.

Some families living near militants or government buildings are leaving their homes to move in with relatives in signs of increasing internal displacement within the small and crowded Gaza Strip.

Israel claims it is carrying out "surgical strikes" and making strenuous efforts to avoid civilian casualties. "It is our intention to avoid what is called collateral damage," Moshe Yaalon, minister for strategic affairs, told a press conference in Jerusalem. "We operate slowly, identify the target and clean the area around it," he added, referring to warnings issued via dropped leaflets and text messages to civilians to stay away from individuals and locations likely to be targeted.

"But when they use civilians as human shields, what is our choice?" he said. "If they position rockets in densely populated areas, such as mosques and schoolyards, we should not be blamed for the outcome."

The World Health Organisation warned that Gaza's medical facilities were being overwhelmed as the number of people injured in air strikes topped 400, about one-third of whom are children.

"Many of those injured have been admitted to hospitals with severe burns, injuries from collapsing buildings and head injuries," the WHO said in a statement issued in Geneva. The UN body appealed for $10m (£6.3m) in immediate international support to help them cope with the casualties.

Sunday's death toll came after a night of heavy bombardment, including repeated and intensive fire from gunboats stationed off the Gaza coast.

Two buildings housing media organisations were struck in the early hours, injuring eight Palestinian journalists including a cameraman who lost a leg.

The al-Shawa building, which includes the studios of al-Quds television, which is associated with Islamic Jihad, and the al-Shuruq building, housing Sky News, al-Arabiya news network, Dubai TV and an office of al-Aqsa TV, which is affiliated with Hamas, were hit.

In a statement, the Israeli Defence Forces said: "A communications antenna used by Hamas to carry out terror activity against the state of Israel, was ... targeted." The IDF denied that journalists were targeted.

Israel opened the Kerem Shalom crossing to Gaza briefly to allow humanitarian aid to enter and 26 patients and their families to leave for medical treatment.