UN Secretary General Ban Ki-moon urged Israel to "exercise maximum restraint" and spare civilian lives in its campaign in Gaza, where the Palestinian death toll on Sunday alone passed 100.
He also condemned the "atrocious action" of Israel in Shejaiya, near Gaza City, where a blistering assault that started at dawn killed 62 people and wounded at least 250.
The United Nations chief made his remarks in Doha at the start of a regional trip aimed at pushing for a ceasefire in the Gaza Strip.
"While I was en route to Doha, dozens more civilians, including children had been killed in Israeli military strikes in Shejaiya. I condemn this atrocious action," he told a news conference.
"Israel must exercise maximum restraint and do far more to protect civilians," he said, adding that it also "must respect the humanitarian law" as it pressed ahead with a deadly offensive that began on July 8.
"Too many innocent people are dying...(and) living in constant fear" in the narrow coastal strip controlled by the Islamist Hamas movement, he said.
At least 438 Palestinians have been killed in Israel's Gaza campaign, more than a third of them women and children, according to medics.
The Israeli army said 13 soldiers had been killed in a series of attacks inside Gaza on Sunday's third day of a major ground operation.
- Bloodshed should stop -
"Gaza is an open wound. We must stop the bleeding now," said Ban, adding that beyond a ceasefire the "root causes of the conflict" must be addressed.
Ban is also travelling to Kuwait City, Cairo, Jerusalem, Ramallah and Amman.
He is due to meet Egyptian President Abdel Fattah al-Sisi in Cairo on Monday to discuss proposals for a Gaza truce.
Egypt's foreign ministry said their talks would "focus on the deteriorating situation in Gaza and the Egyptian initiative for a ceasefire".
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Cairo earlier proposed a halt to the fighting but Hamas rejected it, saying it had not been consulted, and Israel initially accepted it.
Hamas has set conditions for a truce, including the lifting of Israel's eight-year blockade on Gaza, the opening of the Rafah border crossing with Egypt and the release of scores of prisoners Israel rearrested in recent weeks.
Hamas also wants its Turkish and Qatari allies to be involved in any ceasefire negotiations.
Relations between Egypt and Turkey and Qatar are strained over the two countries' support for the banned Muslim Brotherhood movement.
Relations between Egypt and Hamas are also at an all-time low since the military ousted Islamist president Mohamed Morsi last year and outlawed his Muslim Brotherhood, a key Hamas ally.
Qatari Foreign Minister Khalid al-Attiyah said in the joint press briefing with Ban that Doha does not have a separate initiative of its own, as reported earlier.
"We do not claim to have a special initiative. We have just delivered the demands of the Palestinian people," he said.
"It is not important who manages to fulfil the demands of Palestinian people, as long as justice is achieved, even if partially."
- Palestinian Doha talks -
Palestinian President Mahmud Abbas, who is also in Doha, met Ban after he had earlier seen the emir, Sheikh Tamim bin Hamad al-Thani.
He is also due to meet exiled Hamas political chief Khaled Meshaal, who is based in Qatar.
The Hamas leader visited Kuwait briefly for talks on Gaza with the Gulf state's ruler, Emir Sabah al-Ahmad Al-Sabah, the Kuwaiti news agency KUNA said.
Kuwait backs Cairo's initiative to end the bloody conflict, the foreign ministry said in a statement.
It also urged the "international community to put pressure on Israel so that it will stop its aggression" against the Palestinians.
Hamas, meanwhile, has said it has received an invitation to Cairo for ceasefire talks.