The United Nations on Friday fired 12 of its employees in Gaza and began an investigation into them after accusations by Israel that they had helped plan and had participated in the Oct. 7 terrorist assault that left about 1,200 Israelis dead and more than 240 others captured.

The workers, all men employed by the U.N. agency that aids Palestinians, known by the acronym UNRWA, are subject to a criminal investigation, two U.N. officials said.

“UNRWA reiterates its condemnation in the strongest possible terms of the abhorrent attacks,” said Philippe Lazzarini, the head of the agency. “Any UNRWA employee who was involved in acts of terror will be held accountable, including through criminal prosecution.”

In addition, a senior U.N. official, briefed on the accusations, called the allegations “extremely serious and horrific.”

UNRWA has long denied Israel’s claims that it fuels anti-Israeli incitement in Gaza. But it took a sharply different approach to the more serious accusations on Friday, just days after Israel presented them to the agency.

The accusations quickly led the United States to temporarily halt funding to the organization. UNRWA, which provides social services in the Gaza Strip, has been the principal agency overseeing the distribution of aid to Gazans amid a dire humanitarian crisis in the territory that has worsened through months of war since the Oct. 7 attack.

“The United States is extremely troubled by the allegations that 12 UNRWA employees may have been involved in the Oct. 7 Hamas terrorist attack on Israel,” the State Department said in a statement.

The United States is the biggest donor to the agency, providing it with $340 million in 2022 and several hundred million dollars in 2023.

Three Israeli defense officials said military intelligence officers have collected an enormous trove of information after Oct. 7, and in the past two weeks they matched it with a second cache of intelligence that solidified an assessment that the UNRWA employees had been involved in the attack.

UNRWA was created to provide aid to millions of Palestinians across the Middle East whose families fled or were forced from properties during the wars surrounding Israel’s creation in 1948. Since Hamas won Palestinian elections in 2006 and then ousted a rival faction from Gaza a year later, the group, which is considered a terrorist organization by Israel, the United States and other countries, ceded many of its civil responsibilities to UNRWA.

The agency employed about 13,000 workers, most of them Palestinians, before the war began.

Israel and the U.N. have each accused each other of acting in bad faith since Israel launched its war in Gaza following the Oct. 7 Hamas-led assault. The U.N. has accused Israel of slowing the delivery of humanitarian aid to the embattled enclave, and Israel has said the world body has promoted Hamas’s propaganda.

Those recriminations, however, are less politically sensitive than the accusation that humanitarian workers could have engaged in an act of terror, an allegation being taken seriously by the U.N. leadership, the United States and the European Union.

On Oct. 7, Hamas-led assailants raided towns in southern Israel, where they killed, tortured and raped victims. More than 240 people, among them children and old people, were abducted to Gaza as hostages.

Last year, the United Nations General Assembly passed a nonbinding resolution calling on Israel to cease its war in Gaza, and on Friday, the International Court of Justice, the U.N.’s highest judicial body, said Israel must take action to prevent acts of genocide by its forces.

Israel has previously accused UNRWA teachers of telling students in its schools to hate Israel, and it has accused UNRWA employees of collaborating with Hamas. The Trump administration suspended funding to the agency in 2018, but President Biden restored it.

Israeli officials informed the U.N. and the United States earlier this week, according to State Department and U.N. officials.

In recent weeks, Israel has presented new evidence that it says shows UNRWA’s animus toward Israel. Among the recent allegations made by the Israeli military were copies of letters from Hamas’s military wing to the Gaza education ministry asking that teachers be excused from work so they could participate in military training sessions.

The U.S. secretary of state, Antony J. Blinken, spoke on Thursday with the U.N. secretary general, António Guterres, and called for “a thorough and swift investigation,” the State Department said. The department also said it was asking Israel, which initially made the allegation, for more information, and added that “UNRWA plays a critical role in providing lifesaving assistance to Palestinians, including essential food, medicine, shelter, and other vital humanitarian support.”

David Satterfield, the U.S. special envoy for humanitarian affairs, led a group of U.S. officials who met with Maj. Gen. Aharon Haliva of Israel, who showed them the full briefing materials, according to one official who attended the session.

U.N. leaders have repeatedly urged Israel to do more to limit the harm to civilians, and to aid workers, in the conflict. More than 100 U.N. employees have been killed since the start of the war, the most the world body has suffered in a single conflict.

U.N. officials have also warned in stark terms that ordinary residents of Gaza are at risk of starvation and are experiencing a spike in infectious diseases as the weather gets colder.

Josep Borrell Fontelles, the E.U.’s top diplomat and vice president of the European Commission, said he was “extremely concerned” about the allegation that U.N. employees had been involved in the terrorist attacks. He said the commission was in contact with UNRWA and expected it to take immediate measures against the staff involved.

Additional reporting was contributed by Gaya Gupta and Adam Sella.