GENEVA (AP) — The United Nations on Monday expressed growing concerns about humanitarian needs in Palestinian areas as Israel ratchets up a muscular military riposte and lockdown of Gaza, after the weekend attack by Hamas militants who killed and kidnapped hundreds of civilians in Israel.

U.N. Secretary-General António Guterres led the clarion call Monday, putting the focus on civilians in both Israel and Gaza and renewing his condemnation of the fatalities and hostage-takings by Hamas. He also warned of the prospect of more innocent lives lost.

In a statement to reporters in New York, Guterres said that more than 137,000 people in Gaza — or about 6% of its population — were now sheltering in sites run by UNRWA, the aid agency for Palestinians. He cited reports of Israeli missile strikes on places like schools, health facilities and high-rise apartment buildings.

“I am deeply distressed by today’s announcement that Israel will initiate a complete siege of the Gaza Strip, nothing allowed in — no electricity, food, or fuel,” he said.

“The humanitarian situation in Gaza was extremely dire before these hostilities. Now, it will only deteriorate exponentially,” Guterres said.

The U.N. chief was careful not to underplay the devastation and intense suffering among Israelis since Saturday, decrying the launching of “thousands of indiscriminate rockets that have reached central Israel, including Tel Aviv and Jerusalem” and how hundreds of Israelis were killed and many more wounded.

“I recognize the legitimate grievances of the Palestinian people. But nothing can justify these acts of terror and the killing, maiming and abduction of civilians,” Guterres said.

He also noted how Israeli airstrikes had “pounded Gaza” and cited reports of hundreds of Palestinians killed and thousands injured.

Meanwhile, the U.N. Office for the Coordination of Humanitarian Assistance, or OCHA, said damage to water, sanitation and hygiene facilities in Gaza has already hindered services for more than 400,000 people — or about one-sixth of the total population.

As of late Sunday, OCHA said that Israeli authorities had halted supplies of electricity to Gaza, cutting power to no more than four hours per day.

The Gaza Power Plant was the only source of electricity in the area, and it could run out of fuel “within days,” OCHA said in a brief statement on Monday, adding that cash assistance was “urgently needed” for Palestinians.

“Humanitarian relief and essential supplies must be allowed to reach people in need, rapidly and without impediment,” OCHA spokesman Jens Laerke said in a text message. “All parties must ensure freedom of movement for humanitarian workers.”

The agency also noted displacement in Israel because of the violence and damage to civilian homes and infrastructure.

Separately, the World Health Organization said that a 16-year-blockade of Gaza had already left its medical system under-resourced, and the increased hostilities are “compounding an already dire situation.”

The U.N. health agency reported Monday a total of 11 attacks on health care — which included medical facilities, ambulances and care providers — in the first 36 hours of the new conflict in Gaza.

“There is an urgent need to establish a humanitarian corridor for unimpeded, life-saving patient referrals and movement of humanitarian personnel and essential health supplies,” WHO said.

Many humanitarians voiced a simple hope: An end to the violence.

“These unprecedented hostilities — and rapidly mounting death toll — underscore the urgency for all parties to stop the violence and ensure civilians are protected,” says Ana Povrzenic, country director in Palestine at the Norwegian Refugee Council.

In a statement, the Norwegian organization said the humanitarian situation “continues to deteriorate” in Gaza and Israel’s announcement of a “siege” of the area “represents a grave violation of international law.”

Access for humanitarians to Gaza is on many U.N. minds.

“I urge all sides and the relevant parties to allow United Nations access to deliver urgent humanitarian assistance to Palestinian civilians trapped and helpless in the Gaza Strip,” Guterres said. “I appeal to the international community to mobilize immediate humanitarian support for this effort.”