Switzerland says Israel has clearly violated international law by imposing collective punishment on Palestinians over the capture of an Israeli soldier.
Switzerland is the "depository" state of the Geneva Conventions.
They prohibit the deliberate targeting of services essential to the civilian population, like water and electricity.
The statement from Switzerland comes amid growing concern among aid agencies at the deteriorating humanitarian situation in Gaza.
Neutral Switzerland does not often venture into political controversy. But this statement is likely to cause some irritation in Israel and the United States.
"There is no doubt that Israel has not taken the precautions required of it in international law to protect the civilian population and infrastructure"
A spokesman for the Swiss foreign ministry said that, as the depository country for the Geneva Conventions, encouraging respect for international law was a declared aim of Switzerland's foreign policy and the statement should be read in that context.
It says there is "no doubt Israel has not taken the precautions required of it in international law to protect the civilian population and infrastructure.
"The destruction of a power station, the attack on the offices of the Palestinian prime minister, the arbitrary arrests of a large number of democratically-elected representatives of the people and ministers... cannot be justified."
Angry reaction
It is not the first time Switzerland has strayed into the tricky field of Middle East politics.
Earlier this year, the Swiss President Moritz Leuenberger caused anger in Israel when he said the international block on funding to the Palestinian authority was a mistake.
He said the newly elected Hamas government should have been given the chance to prove it could function.
But this latest statement will strike a chord with aid agencies.
They are increasingly concerned about the worsening humanitarian situation in Gaza.
Unicef has seen increased levels of stress and trauma in young children.
The World Health Organization says vital medical supplies are running short and doctors and nurses are having great difficulty even getting to work because of the restrictions on movement.
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