The Al Haj family never heard it coming: An Israeli missile smashed into their home in the middle of the night, destroying the structure and killing eight relatives in a matter of seconds. A survivor said all the dead were civilians.

As Israel intensified its bombardment Thursday of the Gaza Strip in an offensive against the Hamas militant group, with more than 900 targets attacked so far, it said it was doing everything possible to avoid civilian casualties in the crowded urban landscape. The risk of more civilian deaths will remain high, especially if Israel moves in with ground forces.

More than 85 people have been killed, including dozens of civilians, and over 300 wounded since the offensive began Tuesday, Palestinian medical officials said.

Smoke rises following an Israeli strike on Gaza, seen from the Israel-Gaza Border, on Thursday. Israel dramatically escalated its aerial assault in Gaza Thursday hitting hundreds of Hamas targets. (Lefteris Pitarakis/Associated Press)

Undeterred, Hamas militants have fired hundreds of rockets into Israel, including salvos Thursday at the country's two largest cities, Jerusalem and Tel Aviv, that were intercepted by the rocket-defence system known as the Iron Dome.

President Barack Obama called Israeli Prime Minister Benjamin Netanyahu and lent his support to Israel's efforts to defend itself from the rocket fire, but he also urged both Israel and the Palestinians to protect civilians and restore calm.

The White House said the U.S. was willing to "facilitate a cessation of hostilities," potentially along the lines of a 2012 cease-fire that Egypt and former Secretary of State Hillary Rodham Clinton helped broker.

UN Security Council meets to discuss crisis

At an emergency meeting of the UN Security Council, Secretary-General Ban Ki-moon expressed deep concern about the threats to civilians in Israel and Gaza, and urged an immediate cease-fire.

"It is unacceptable for citizens on both sides to permanently live in fear of the next aerial attack," Ban said. "My paramount concern is the safety and well-being of all civilians, no matter where they are."

Israel continues to mass troops, tanks and armoured personnel carriers outside of the central Gaza Strip. (Baz Ratner/Reuters)

He condemned the "indiscriminate" rocket fire at Israel. "But I am also concerned at the many Palestinian deaths and injuries as a result of Israeli operations," he said. "Once again, Palestinian civilians are caught between Hamas's irresponsibility and Israel's tough response."

Israel's UN ambassador, Ron Prosor, pulled out a cellphone during the meeting and played a recording of an air-raid siren as he described the difficult circumstances of people living within rocket range. His Palestinian counterpart, Riyad Mansour, decried the Israeli "barrage of death, destruction and terror."

Ban has been a key player in diplomatic efforts to halt the hostilities. Egypt, historically a mediator between Israel and the Palestinians, has said it is in touch with both sides.

Neither side has shown much interest in halting the fighting. With rockets continuing to fly, Israel has been massing forces along the Gaza border in preparation for a possible ground invasion.

"So far the campaign is going as planned," Netanyahu said in a statement broadcast on national television. "But we are expecting more stages later. So far we have severely hit Hamas and other terrorists and we will deepen the strike against them as long as the campaign continues."

He said Israel was making "every effort" to avoid harming civilians and accused Hamas of endangering the Palestinian public by "hiding behind Palestinian civilians."

Residents in the crowded Khan Younis refugee camp in southern Gaza were at a loss to explain why the Al Haj family home was targeted in the attack just before 2 a.m. Thursday. The blast killed Mahmoud Al Haj, his wife, Basmah, and six of their children.

Israel says Hamas militants hiding among civilians

Yasser Al Haj, who was not at home in the refugee camp, was the only relative to survive.

"I want to see my family," he wailed. "Where is the house? Where is my family?"

A woman takes a photo with her mobile phone of a car damaged when the remains of a rocket intercepted by Israel landed in a Tel Aviv neighbourhood Thursday. (Finbarr O'Reilly/Reuters)

Neighbour Iyad Hamad decried the deaths of "children, women and old people."

"There can't be more oppression than this in the world," he said. "Can't they see what is happening to the people here?"

It was not clear who was the target of the airstrike, or whether the family had been warned.

Lt. Col. Peter Lerner, a military spokesman, said the incident was under investigation.

Israel has been taking precautions to protect civilians. He said when Israel identifies a home used by Hamas as a "command and control center," it calls the inhabitants and orders them to leave. It then fires a "non-explosive munition" at the roof as a warning and looks for people to leave. Only then does it destroy the structure.

But he said if a militant is actively using a house to carry out attacks, he becomes a legitimate target. "If you use civilian premises to perpetrate attacks, you are putting yourself and the people around you in a state of danger," Lerner said.

Israel says Hamas intentionally uses civilian areas, including homes, mosques and schools, for cover during fighting. "Hamas ... intentionally embeds itself in civilian populations," Lerner said.

A number of the nearly 30 women and children to die in the offensive are believed to have been killed along with relatives who were targeted by Israel.