The UN's top human rights official has called for an investigation into Israeli air strikes on Gaza, on the grounds that the targeting of Palestinian homes – resulting in a high death toll among civilians, particularly children – could violate international law.

The warning from Navi Pillay, the UN high commissioner for human rights, came on the fourth day of Israel's bombing of the Gaza Strip and a rocket barrage of Israel by Islamic militants.

However, the Israeli prime minister, Binyamin Netanyahu, said his government would not be deflected by criticism from abroad, refusing to rule out a ground offensive and vowing there would be more air strikes. So far more than 100 Palestinians have been killed, mostly civilians, including at least 23 children. More than 670 have been injured. There have as yet been no Israeli fatalities.

Pillay said her office had received "deeply disturbing reports that many of the civilian casualties, including children, occurred as a result of strikes on homes" in Gaza. "Such reports raise serious doubt about whether the Israeli strikes have been in accordance with international humanitarian law and international human rights law."

Pillay added that the "indiscriminate firing of rockets from Gaza" could also constitute a breach.

"Every alleged breach of international law must be promptly, independently, thoroughly and effectively investigated, with a view to ensuring justice and reparations for the victims," she said.

Netanyahu shrugged off foreign criticism and said the Israeli bombing would continue unabated. "No international pressure will prevent us from acting with all power," he said, claiming to have had "good conversations" with several world leaders in recent days, including Barack Obama and European heads of government.

He claimed Israeli planes and drones had attacked more than 1,000 targets in Gaza so far this week, adding, "there are still more to go". The Israeli prime minister said Israel had already struck Gaza with twice the force used during the last offensive of its kind in 2012, and he would not rule out following the air campaign with an incursion by ground troops. "We are weighing all possibilities and preparing for all possibilities," he said.

Israeli forces have been warning of imminent air strikes with the use of mobile phone texts and warning shots on the roofs of targeted buildings, but children are believed to constitute such a high proportion of the dead partly because they are often the most afraid to leave their homes while their neighbourhoods are being bombed.

When Pillay visited Gaza and Israel in 2011 in the wake of a similar exchange of fire, she said that both Hamas and the Israeli government should be held liable for war crimes and that Israeli forces had committed crimes against humanity.

"Israel, Hamas, and Palestinian armed groups in Gaza have been down this road before, and it has led only to death, destruction, distrust and a painful prolongation of the conflict," Pillay said on Friday. UN officials said the current air strikes would have to be investigated further before a judgment on potential war crimes could be made.

Israel was reported to have been hit by 809 rockets and 61 mortars from Gaza this week. While nobody has been killed, according to local media reports, nine Israeli civilians have so far been hurt in the scramble to take cover after air-raid sirens.

Jens Laerke, spokesman of the UN Office for the Coordination of Humanitarian Affairs (OCHA), told journalists: "More than 340 housing units in Gaza have been severely damaged or completely destroyed. As a result, more than 2,000 people have been displaced.

"Our aid workers on the ground report that people in Gaza are gripped by fear, the streets are empty and the shops are closed."