The World Health Organization chief on Sunday urged Hamas to release all civilian hostages, as he warned that the war between Israel and the Islamist group would only bring destruction and horror.
WHO director-general Tedros Adhanom Ghebreyesus said he was gravely concerned by Israeli attacks in which innocent Palestinian civilians and children were "paying the price".
The conflict "is an awful reminder of how quickly the health of millions of people can be put at risk", the UN agency's leader said, in comments circulated by the WHO headquarters in Geneva.
"War will bring nothing but destruction and horror," he told the World Health Summit in Berlin, speaking from Manila.
Israel declared war on Hamas last Sunday, a day after waves of the militant group's fighters broke through the heavily fortified border and shot, stabbed and burned to death more than 1,400 people, most of them civilians.
The subsequent relentless bombing has flattened neighbourhoods and left at least 2,670 people dead in the Gaza Strip, the majority ordinary Palestinians.
The Israeli military said Sunday it had confirmed 155 people were being held hostage by Hamas since the group staged its deadly attack last week.
"WHO calls on Hamas to release civilian hostages, and we continue to appeal to Israel to abide by its obligations under international law to protect civilians and health facilities," said Tedros.
He branded the Hamas attacks "unjustified and horrific", saying they were "barbaric and should be condemned".
"I'm also gravely concerned about Israeli attacks on Palestinian civilians. Innocent civilians and children are paying the price," he added.
Israel's army has told people in the north of the Gaza Strip -- nearly half of its 2.4-million population -- to head south to safety, ahead of an expected ground offensive.
"Asking 1.1 million people to move from north to south Gaza in such a short window will create a humanitarian tragedy," said Tedros.
"The forced evacuation of patients and health workers will further worsen the humanitarian and public health catastrophe."
The WHO is calling for the restoration of electricity and water in the Gaza Strip, and for conditions to allow "the immediate and safe delivery of food, medical supplies, and other humanitarian aid", said Tedros.