"We have received disturbing reports that many of the civilian casualties, including of children, occurred as a result of strikes on homes," said spokeswoman Ravina Shamdasani.
"Such reports raise doubts about whether the Israeli air strikes have been in accordance with international humanitarian law and international human rights law," she told reporters.
International humanitarian law is UN-speak for the laws of war, and Shamdasani said targeting homes was a violation unless the buildings were being used for military purposes.
"Even when a home is identified as being used for military purposes, any attack must be proportionate, offer a definite military advantage in the prevailing circumstances at the time, and precautions must be taken to protect civilians," she added.
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Israel has accused the Islamist movement Hamas and other Palestinian militants who have fired hundreds of rockets at its territory of deliberately placing military installations in densely-populated Gaza in order to use civilians as human shields.
"This is a call to the armed groups on the Palestinian side," she added, condemning their 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. 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," said Shamdasani.
"It is incumbent on both parties to ensure that their military operations respect the law, no matter what the obstacles and no matter what the difficulties.