A National Security Memorandum, known as NSM-20, that Biden issued in February requires the State Department to report to Congress by May 8 on whether it finds credible Israel's assurances that its use of U.S. weapons does not violate U.S. or international law.

Four sources said on Tuesday the administration had informed congressional committees that it would not make the deadline, but hoped to present its findings within days. Two congressional aides said they had no indication the delay was tied to political concerns.

Reuters reported last month that some senior U.S. officials do not find Israel's assurances credible. The Reuters report, along with investigations by outside organizations like Amnesty International, has prompted some lawmakers to call on the Biden administration not to tilt the report toward Israel.

"I've had a lot of conversations... with folks in the administration, really urging them to make sure that this report is credible, that it's seen to be based on facts and law and not based on what they would wish it would be," Democratic Senator Chris Van Hollen told reporters.

Washington's provision of military assistance to Prime Minister Benjamin Netanyahu's government has prompted protests across the U.S. demanding that universities and Biden withdraw support for Israel, including sending weaponry.

'NOT CREDIBLE'

In Washington, many of Biden's fellow Democrats have called for a shift in long-standing U.S. policy of providing unconditional military support to the Jewish state.

Israel’s assurances of compliance with U.S. law are "not credible," said Representative Jason Crow, who last week organized a letter to Biden from more than 80 Democratic lawmakers saying there is sufficient evidence that Israel has violated international law and obstructed U.S. aid deliveries to Gaza.

On Tuesday, sources told Reuters that Biden's administration has been holding up certain arms shipments to Israel, in what two of the sources said was an apparent political message to the U.S. ally.

State Department spokesperson Matthew Miller told a news briefing on Tuesday the NSM-20 report was not yet finished but the department was working "very hard" to complete it. "It's possible it slips just a little bit but we're still, at this point, trying to get it done by tomorrow," he said.

The memorandum bars any recipient of U.S. military assistance from restricting the delivery of humanitarian aid.

The report deadline comes amid concern about famine in Gaza and calls from the United States, other governments and international bodies for Israel to hold off from a big offensive against Rafah, a city in Gaza Israel calls Hamas fighters' last stronghold but is also the refuge of more than one million displaced Palestinian civilians.