Palestinian Authority President Mahmoud Abbas said on Wednesday Gaza has become a "humanitarian disaster area" and urged the UN and international aid agencies to assume responsibility for meeting the needs of the Palestinians in the coastal enclave.

"I call for the use of all tools available within the United Nations to provide relief and help to the Palestinian people during this grave humanitarian crisis," Abbas said in a letter to UN Secretary-General Ban Ki-moon, according to official Palestinian news agency Wafa.

Both US President Barack Obama and the UN Security Council have called for an immediate ceasefire to allow relief to reach Gaza's 1.8 million Palestinians, followed by negotiations on a more durable end to hostilities.

Earlier on Wednesday, health officials in Gaza said Israeli shelling killed at least 15 Palestinians sheltering in a UN-run school and another 17 near a street market on Wednesday.

However, Deputy Foreign Minister Tzahi Hanegbi said Israel has ample evidence that Hamas is using UNRWA schools and facilities for terrorist purposes.

Speaking at a press briefing in Jerusalem, he pointed out that on three separate occasions UNRWA itself has admitted that rockets were hidden in the basement of their facilities. “Who knows how many were not found,” he said, adding that this phenomenon was “totally unacceptable.”

Israel's security cabinet decided to continue its offensive in the enclave and there was no sign of a halt to a 23-day conflict.

Herb Keinon contributed to this report.

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