The U.N. secretary general, António Guterres, on Tuesday sharply criticized Israel’s renewed blockade of aid to Gaza as a violation of its obligations under international law and said it had opened “the floodgates of horror.”

“Gaza is a killing field and civilians are in an endless death loop,” Mr. Guterres said in prepared remarks to journalists at United Nations headquarters in New York.

“More than an entire month has passed without a drop of aid into Gaza,” he said. “No food. No fuel. No medicine. No commercial supplies. As aid has dried up, the floodgates of horror have reopened.”

It was some of the secretary general’s strongest language about Israel since the country’s war with Hamas began. His words reflected what senior U.N. officials say is a mounting sense of desperation over the failure of the international community — including Israel’s closest ally, the United States — to constrain Israel’s military conduct in Gaza.

The Israeli government rejected Mr. Guterres’s criticisms.

“As always, you don’t let the facts get in the way when spreading slander against Israel,” the spokesman for Israel’s Foreign Ministry, Oren Marmorstein, said on social media. “There is no shortage of humanitarian aid in the Gaza Strip — over 25,000 aid trucks have entered the Gaza Strip in the 42 days of the cease-fire. Hamas used this aid to rebuild its war machine. Yet, not a word in your statement about the imperative for Hamas to leave Gaza.”

As an occupying power in Gaza, Mr. Guterres said, Israel has “inescapable obligations” under the Geneva Conventions to ensure delivery of food and medicine and to maintain hospitals and public health services.

“None of that is happening today,” he said.

And new aid delivery mechanisms proposed by Israel, Mr. Guterres said, risk “further controlling and callously limiting aid down to the last calorie and grain of flour,” he said. The U.N. will not participate in any arrangement that does not respect basic humanitarian principles of independence and neutrality, he said. He also said that assertions that Gaza had enough food to feed its entire population were far from the reality on the ground, where commodities are running extremely low.

“The current path is a dead end — totally intolerable in the eyes of international law and history,” Mr. Guterres said.

A day after meeting with the families of Israelis taken hostage in the Oct. 7, 2023, Hamas attack that set off the war, Mr. Guterres also called for the hostages’ immediate and unconditional release. “I condemned Hamas for their brutal kidnapping and horrible treatment,” he said on social media.

The meeting with the families took place the same day that the heads of the U.N.’s six biggest aid agencies issued a rare joint statement. “We are witnessing acts of war in Gaza that show an utter disregard for human life,” it said.

In just the first week after the cease-fire broke down, more than a thousand children were reported killed or injured in Gaza, said the agency chiefs, including those responsible for food, children, health and humanitarian aid. It was the highest one-week death toll for children there of the past year, they said.

The statement was issued amid international calls for an independent investigation into the deaths of 15 Palestinian Red Crescent staff members and aid workers shot by Israeli forces last month in an attack that U.N. human rights chief, Volker Türk, said added to concerns over the commission of war crimes by the Israeli military.

The International Criminal Court has already issued arrest warrants for Israel’s prime minister, Benjamin Netanyahu, and for a Hamas leader, accusing them of war crimes, though Mr. Netanyahu has been traveling abroad over the past week anyway. On Monday, he met with President Trump at the White House.

On Tuesday, the U.N. humanitarian aid coordinator, Tom Fletcher, chided the Security Council for doing too little to protect civilians and uphold international law, pointing to the conflicts not just in Gaza but also Ukraine, Sudan and Myanmar.

“Not only are we not standing robustly for international law, but in some cases we are supporting its debasement,” Mr. Fletcher said. “That’s the common thread that links these conflicts.”

Johnatan Reiss contributed reporting.