UNRWA did not say how many employees had come under suspicion; the United States put the number at 12. The agency employs around 13,000 people in Gaza, where it provides food assistance, health care and education.

The State Department said it was pausing new funding for the agency while it reviewed the allegations and the steps the United Nations was taking to address them.

The United States is UNRWA’s largest donor; Secretary of State Antony Blinken has described the agency’s mission as “essential.” Any suspension of U.S. aid, even if only temporary, could weaken the agency’s ability to respond to what aid workers have described as dire humanitarian need. More than 93 percent of the population faces “crisis levels of hunger,” and 1.9 million people have sought refuge in UNRWA shelters.

“These shocking allegations come as more than 2 million people in Gaza depend on lifesaving assistance that the Agency has been providing since the war began,” Lazzarini said. “Anyone who betrays the fundamental values of the United Nations also betrays those whom we serve in Gaza, across the region and elsewhere around the world.”

Israel’s campaign to eradicate Hamas from Gaza has caused the deaths of more than 26,000 Palestinians, according to the Hamas-run Gaza ministry of health. Most were civilians.

Aid officials say their personnel have been targeted while conducting lifesaving activities. One hundred and fifty-two UNRWA staffers have been killed, the agency says, and at least 357 displaced civilians have been killed while sheltering in its facilities.

The agency made the allegations, the termination of its workers and the investigation public hours after the International Court of Justice ordered Israel to do more to prevent the deaths of civilians in Gaza and allow more aid into the besieged enclave.

A State Department spokesman said the United States was “extremely troubled” by the allegations and welcomed the investigation. “The United States has reached out to the Government of Israel to seek more information about these allegations, and we have briefed Members of Congress,” spokesman Matt Miller said in a statement. “We will remain in close contact with the United Nations and Government of Israel regarding this matter.”

A spokesman for Israel’s Ministry of Foreign Affairs said it expected an “urgent examination” of the allegations. He did not provide further information.

“It is crucial that UNRWA conducts a thorough internal investigation into the activities of Hamas operatives and other terrorist elements within its ranks to ensure that the humanitarian activities of the organization are not exploited for malicious purposes,” spokesman Lior Haiat said.

The Biden administration has been Israel’s most important ally throughout the war. It has expedited the transfer of American weapons while urging Israel to do more to limit civilian casualties. Critics of Israel’s military operation noted the timing of the allegations, which were made public on a day when the ICJ’s ruling would otherwise have dominated the news from Gaza.

The United States has given UNRWA more than $1 billion since 2021. A National Security Council spokesperson said in November that the State Department maintained “rigorous oversight” over the agency’s policies, programs, and finances.

Risk-mitigation measures, he said, included closely monitoring the organization’s activities through partner vetting, anti-terrorism certifications, and close cooperation with Israeli authorities.

“UNRWA is an essential component to any plan that seeks to maintain stability in the region and protect civilians,” he said.

Jeff Stein and John Hudson in Washington contributed to this report.