The United Nations, aid groups and public health experts expressed growing concerns Monday about humanitarian needs in Palestinian areas as Israel ratchets up a muscular military riposte and a 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, endeavoring to put 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.