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.