The Biden administration on Friday said Israel’s use of US supplied weapons may have violated international humanitarian law during its military operation in Gaza, in its strongest criticism to date of Israel.

But the administration stopped short of a definitive assessment, saying that due to the chaos of the war in Gaza it could not verify specific instances where use of those weapons might have been involved in alleged breaches.

The assessment came in a 46-page unclassified State Department report to Congress required under a new National Security Memorandum (NSM) that President Joe Biden issued in early February.

The findings risk further souring ties with Israel at a time when the allies are increasingly at odds over Israel’s plans to strike Rafah, a move Washington has repeatedly warned against.

The Biden administration has already put a hold on one package of arms in a major policy shift and said the US was reviewing others even as it reiterated long-term support for Israel.

The State Department’s report included contradictions: It listed numerous credible reports of civilian harm and said Israel did not at first cooperate with Washington to boost humanitarian assistance to the enclave. But in each instance it said it could not make a definitive assessment whether any breaches of law had occurred.

“Given Israel’s significant reliance on US-made defense articles, it is reasonable to assess that defense articles covered under NSM-20 have been used by Israeli security forces since October 7 in instances inconsistent with its IHL obligations or with established best practices for mitigating civilian harm,” the State Department said in the report.

“Israel has not shared complete information to verify whether US defense articles covered under NSM-20 were specifically used in actions that have been alleged as violations of IHL or IHRL in Gaza, or in the West Bank and East Jerusalem during the period of the report,” it said.

Because of that, the administration said it still finds credible Israel’s assurances that it is using US weapons in accordance with international law.

Democratic Senator Chris Van Hollen said the administration had “ducked all the hard questions” and avoided looking closely at whether Israel's conduct should mean military aid is cut off.

“This report contradicts itself because it concludes that there are reasonable grounds to believe violations to international law have occurred, but at the same time that says they’re not finding non compliance,” he told reporters.

More than 34,000 Palestinians have been killed in Israel’s seven-month-old assault on the Gaza Strip, say health officials in the Hamas-ruled enclave. The war began when Hamas militants attacked Israel on Oct. 7, killing 1,200 people and abducting

252 others, of whom 133 are believed to remain in captivity in Gaza, according to Israeli tallies.

Israel’s military conduct has come under increasing scrutiny with the soaring death toll and the level of devastation in the Gaza Strip.

US officials at the State Department have been divided over the issue. Reuters reported that officials in at least four bureaus inside the agency have raised serious concerns over Israel's conduct in Gaza, laying out specific examples where the country might be in breach of the law.

Rights group Amnesty International in late April said US -supplied weapons provided to Israel have been used in “serious violations” of international humanitarian and human rights law, detailing specific cases of civilian deaths and injuries and examples of use of unlawful lethal force.

The US government reviewed numerous reports that raise questions about Israel’s compliance with its legal obligations and best practices for mitigating harm to civilians, the report said.

Those included Israeli strikes on civilian infrastructure, strikes in densely populated areas and others that call into question whether “expected civilian harm may have been excessive relative to the reported military objective.”

According to the report released Friday, in the period after Oct. 7 Israel “did not fully cooperate” with US and other international efforts to get humanitarian aid into Gaza. But it said this did not amount to a breach of a US law that blocks the provision of arms to countries that restrict US humanitarian aid.

It said Israel had acted to improve aid delivery since Biden warned Prime Minister Benjamin Netanyahu in a call early last month that Washington would withhold some arms supplies if the humanitarian situation did not improve.

The report, Washington has decided to declassify, said individual violations do not necessarily disprove Israel’s commitment to international humanitarian law, as long as it takes steps to investigate and hold violators accountable.

“Israel’s own concern about such incidents is reflected in the fact it has a number of internal investigations underway,” the report said. A senior State Department official confirmed that none of those investigations had yet led to prosecutions.

It also has compiled numerous instances in which humanitarian workers have been killed and military operations had taken place in protected sites but again said it was not able to reach definitive conclusions on whether US weapons were used in these occasions.