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.

Guterres said in a statement that over 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, stressing the need for medical equipment, food, fuel and other humanitarian supplies and aid providers.

Meanwhile, the U.N. Office for the Coordination of Humanitarian Assistance 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 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,” said OCHA in a brief statement on Monday, adding that cash assistance was “urgently needed” for beleaguered 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.”

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