The Biden administration believes that Israel has most likely violated international standards in failing to protect civilians in Gaza but has not found specific instances that would justify the withholding of military aid, the State Department told Congress on Friday.

In the administration’s most detailed assessment of Israel’s conduct in Gaza, the State Department said in a written report that Israel “has the knowledge, experience and tools to implement best practices for mitigating civilian harm in its military operations.”

But it added that “the results on the ground, including high levels of civilian casualties, raise substantial questions” as to whether the Israel Defense Forces are making sufficient use of those tools.

Even so, the report — which seemed at odds with itself in places — said the U.S. had no hard proof of Israeli violations. It noted the difficulty of collecting reliable information from Gaza, Hamas’s tactic of operating in civilian areas, and the fact that “Israel has not shared complete information to verify” whether U.S. weapons have been used in specific incidents alleged to have involved human rights law violations.

The report, mandated by President Biden, also makes a distinction between the general possibility that Israel has violated the law and any conclusions about specific incidents that would prove it. It deems that assurances Israel provided in March that it would use U.S. arms consistent with international law are “credible and reliable,” and thus allow the continued flow of U.S. military aid.

The conclusions are unrelated to Mr. Biden’s recent decision to delay the delivery to Israel of 3,500 bombs and his review of other weapons shipments. The president has said those actions were in response to Israel’s stated plans to invade the southern Gaza city of Rafah.

The report said its findings were hampered in part by the challenges of collecting reliable information from the war zone and the way Hamas operates in densely populated areas. It also stressed that Israel has begun pursuing possible accountability for suspected violations of the law, a key component in the U.S. assessment about whether to provide military aid to allies accused of human rights violations.

Israel has opened criminal investigations into the conduct of its military in Gaza, the report said, and the Israel Defense Forces “are examining hundreds of incidents” that may involve wartime misconduct.

The report also did not find that Israel had intentionally obstructed humanitarian aid into Gaza.

While it concluded that both “action and inaction by Israel” had slowed the flow of aid into Gaza, which is desperately short of necessities like food and medicine, it said that “we do not currently assess that the Israeli government is prohibiting or otherwise restricting the transport or delivery of U.S. humanitarian assistance” into the territory.

Such a finding would have triggered a U.S. law barring military aid to countries that block such assistance.

Brian Finucane, a former State Department lawyer now with International Crisis Group, said the report “bends over backwards” to avoid concluding that Israel violated any laws, a finding that would place major new pressure on Mr. Biden to restrict arms to the country.

Mr. Finucane, a critic of Israel’s military operations, said that the report was “more forthcoming” than he had expected, but that he still found it “watered down” and heavily “lawyered.”

The findings further angered a vocal minority of Democrats in Congress who have grown increasingly critical of Israel’s conduct in Gaza. They argue that Israel has indiscriminately killed civilians with American arms and intentionally hindered U.S.-supplied humanitarian aid.

Either would violate U.S. laws governing arms transfers to foreign militaries, as well as international humanitarian law, which is largely based on the Geneva Conventions.

The report did not define the meaning of its other criteria for Israel’s actions, “established best practices for mitigating civilian harm,” though it cited Defense Department guidelines on the subject released last year, which include some measures “not required by the law of war.”

“If this conduct complies with international standards, then God help us all,” Senator Chris Van Hollen, Democrat of Maryland, told reporters after the report’s release. “They don’t want to have to take any action to hold the Netanyahu government accountable for what’s happening,” he added, referring to Israel’s prime minister, Benjamin Netanyahu.

Critics of Mr. Biden’s continuation of most military support to Israel had hoped that he would use the report as a justification for further restricting arms deliveries to the country. The United States provides Israel with $3.8 billion in annual military aid, and Congress last month approved an additional $14 billion in emergency funding.

Mr. Biden ordered the report with a national security memorandum known as NSM-20. It requires all recipients of U.S. military aid engaged in conflict to provide the United States with written assurances that they will comply with international law and not hinder the delivery of humanitarian aid provided by or supported by the U.S. government.

The report called on the secretary of state and the defense secretary to assess “any credible reports or allegations” that American weapons might have been used in violation of international law.

Since the president’s memorandum was issued, an independent task force formed in response issued a lengthy report citing dozens of examples of likely Israeli legal violations. That report found what it called Israel’s “systematic disregard for fundamental principles of international law,” including “attacks launched despite foreseeably disproportionate harm to civilians” in densely populated areas.

In a statement following the State Department report, the task force called the U.S. document “at best incomplete, and at worst intentionally misleading in defense of acts and behaviors that likely violate international humanitarian law and may amount to war crimes.”

“Once again, the Biden Administration has stared the facts in the face — and then pulled the curtains shut,” said the task force’s members, who include Josh Paul, a former State Department official who in October resigned in protest over U.S. military support for Israel.

The State Department report showed clear sympathy for Israel’s military challenge, repeating past statements by the Biden administration that Israel has a “right to defend itself” in the wake of the Oct. 7 Hamas attacks. It also noted that military experts call Gaza “as difficult a battlespace as any military has faced in modern warfare.”

“Because Hamas uses civilian infrastructure for military purposes and civilians as human shields, it is often difficult to determine facts on the ground in an active war zone of this nature and the presence of legitimate military targets across Gaza,” it said.

Even so, it singled out numerous specific incidents where Israel’s military had killed civilians or aid workers, the latter of which it called a “specific area of concern.”

Those episodes include the killing of seven World Central Kitchen workers in April. The report noted that Israel has dismissed officers and reprimanded commanders involved in that attack, which Israel has called “a grave mistake,” and is considering prosecutions.

Other episodes it cited included airstrikes on Oct. 31 and Nov. 1 on the crowded Jabaliya refugee camp, which reportedly killed dozens of civilians, including children. It noted Israel’s claim that it had targeted a senior Hamas commander and underground Hamas facilities at the site, and that its munitions had “led to the collapse of tunnels and the buildings and infrastructure above them.”

And while the report did not find that Israel had intentionally hindered the delivery of humanitarian aid, it listed several examples of ways in which its government had “a negative effect” on aid distribution. They included “extensive bureaucratic delays” and what it called the active involvement of some senior Israeli officials in protests or attacks on aid convoys.

The report was delivered to Congress two days after the deadline set by Mr. Biden’s February memorandum, arriving late on a Friday afternoon — the time of choice for government officials hoping to minimize an announcement’s public impact. Earlier that day, a White House spokesman, John F. Kirby, denied that the delay had any “nefarious” motive.