Israeli prime minister Benjamin Netanyahu says the Jewish state has hit more than 1,000 targets during its four-day-old offensive against Gaza militants, and that "there are still more to go".

It comes as the United Nations says Israel could be violating the laws of war by bombing Palestinian homes in Gaza, with the death toll from Israeli strikes now more than 100.

Israel, however, says it will not bow to international pressure to end the airstrikes, despite an offer from US president Barack Obama to help negotiate a ceasefire with militants.

When asked if Israel might move from the mostly aerial attacks of the past four days into a ground war in Gaza to stop militant rocket fire, Mr Netanyahu told reporters that "we are weighing all possibilities and preparing for all possibilities".

"No international pressure will prevent us from acting with all power," he said.

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A health ministry spokesman in Gaza has put the death toll from Israel's Operation Protective Edge at 105, with 154 people wounded.

"What's the point? What for?" said a young man visiting a friend at Shifa Hospital, who was injured in a bomb blast.

"We are civilians on our way to get food. So why did they hit so many people?"

Since the operation began, militants have fired at least 500 mortar rounds and rockets that struck Israel, while another 130 rockets were intercepted by the Iron Dome missile defence system, an Israeli army spokeswoman said Friday.

No Israelis have been killed, but Mr Netanyahu says the Israeli airstrikes "will continue until we are certain that quiet returns to Israeli citizens".

On the Gaza border, Israeli troops and tanks are standing by, maintaining the threat of a ground invasion.

"We are in the midst of an assault," army chief lieutenant-general Benny Gantz said, "and we are prepared to expand it as much as is required, to wherever it's required, with whatever force will be required and for as long as will be required."

UN says Israel should not be bombing homes

"We have received disturbing reports that many of the civilian casualties, including of children, occurred as a result of strikes on homes," Ravina Shamdasani, spokeswoman for the UN's human rights office, said.

"Such reports raise doubts about whether the Israeli air strikes have been in accordance with international humanitarian law and international human rights law."

No let up: Israeli prime minister Benjamin Netanyahu. ( Reuters: Gali Tibbon )

Ms Shamdasani said targeting homes was a violation unless the buildings were being used for military purposes, and even then restrictions apply.

"In case of doubt, buildings that are ordinarily used for civilian purposes, such as homes, are presumed not to be legitimate military targets," she said.

Israel has accused Palestinian militants, including the Islamist group Hamas, of deliberately placing military installations in the densely populated Gaza in order to use civilians as human shields.

Ms Shamdasani condemned militant attacks on Israeli civilian areas, which have injured less than a dozen people.

"On the Israeli side, however, their responsibility in international law is very specific," she said.

"If there is even an iota of a doubt, homes are not legitimate military targets. And if these homes are being used for military purposes, attacks must be proportionate, and precautions must be taken."

Turkey's PM lashes out at Israeli 'lies' over Gaza

Meanwhile, Turkish prime minister Recep Tayyip Erdogan has accused Israel of a policy "based on lies" over its aerial bombardment of Gaza, in his latest vehement criticism of the Jewish state.

Speaking to supporters in Istanbul, Mr Erdogan pointed out that 100 Palestinians had now been killed in the Israeli campaign while no Israelis had died due to Palestinian attacks.

"They say [Hamas] is firing rockets. But is there anybody who died?" he asked.

"The number of Palestinians that [Israel has] killed is now 100. [Israel's] life is based on lies. They are not honest.

"We cannot take the side of the cruel."

Mr Erdogan in 2009 famously walked out of the World Economic Forum in Davos during a previous Israeli assault on Gaza and relations between Israel and Turkey have become even frostier over the last years.

AFP/Reuters