(WASHINGTON) -- The Biden administration opted to pause a shipment of some 3,500 bombs to Israel last week because of concerns the weapons could be used in Rafah where more than one million civilians are sheltering "with nowhere else to go," a senior administration official told ABC News.
Other weapon transfers from the U.S. to Israel -- including the sale of Joint Direct Attack Munition, or JDAM kits -- are being closely scrutinized as part of a larger review of U.S. military aid to Israel that began last April, the official said.
The decision to pause the shipment and consider slow-walking others is a major shift in policy for the Biden administration and the first known case of the U.S. denying its close ally military aid since the Israel-Hamas war began.
Asked about the reporting at a Senate hearing on Wednesday, Defense Secretary Lloyd Austin publicly confirmed the U.S. has paused a munitions shipment and that a major Israeli operation in Rafah could change the U.S. calculus on security assistance to Israel.
He's the first senior administration official to confirm the U.S. pause on military aid but he added that no "final determination" has been made.
"We are currently reviewing some near-term security assistance shipments in the context of unfolding events in Rafah," Austin told a Senate Appropriations subcommittee, emphasizing that supplemental funding Congress recently passed is not in jeopardy.
"We've been very clear," he said, "from the very beginning that that Israel shouldn't launch a major attack and Rafah without accounting for and protecting those civilians that are hitting that battlespace. And, again as we have assessed the situation, we paused one shipment of high payload munitions."
At the same time, Austin insisted that the U.S.-Israel alliance is "ironclad."
The Biden administration has been reluctant in the past to withhold weapons from Israel despite policy differences because such contracts are typically years in the making, and withholding aid is unlikely to influence Israeli policy decisions in the near term. At the same time, U.S. officials worried that delaying future weapons shipments could put Israel's defense -- a strategic priority for the U.S. -- at risk.
According to the senior administration official, who spoke on condition of anonymity Tuesday in order to discuss a sensitive policy decision that hadn't yet been publicly announced, the move came because U.S.-Israeli talks on the humanitarian needs in Rafah "have not fully addressed our concerns."
"As Israeli leaders seemed to approach a decision point on such an operation, we began to carefully review proposed transfers of particular weapons to Israel that might be used in Rafah," the official said in a written statement provided to ABC News.
More than half of the shipment that was paused last week consisted of 2,000-pound bombs. The remaining 1,700 bombs were 500-pound explosives, the official said.
"We are especially focused on the end-use of the 2,000-pound bombs and the impact they could have in dense urban settings as we have seen in other parts of Gaza," the official told ABC News. "We have not made a final determination on how to proceed with this shipment."
Other cases that remain under review include JDAM kits, which enable precision targeting of bombs.
Several other U.S. officials confirmed the policy decision earlier on Tuesday, the same day Israel began what its officials called a "precise" operation in Rafah. U.S. officials said they did not believe those operations were the beginning of the larger-scale invasion that Israel has been planning for weeks; that timeline remains uncertain, they say.
Israeli Prime Minister Benjamin Netanyahu has said operations in Rafah are inevitable and necessary to eliminate Hamas.
The White House declined to discuss specifics at a press briefing, instead pointing to the overall U.S. support for Israel.
"Our commitment to Israel's security remains ironclad," said White House spokesman John Kirby. "We don't, as a matter of course, talk about individual shipments one way or the other. But again, nothing's changed about our commitment to Israel security."
Biden has faced criticism from some Democrats and other voters who say he's not doing enough to stop the mounting death toll of civilians in Gaza. At the same time, support for Israel's operations in Gaza remains strong among Republicans.
On Tuesday, top GOP lawmakers were quick to criticize the delay in arms shipments to Israel that were already approved by Congress.
"This is not the will of Congress," said House Speaker Mike Johnson. "This is an underhanded attempt to withhold aid, without facing accountability. It's undermining what Congress intended."
In a letter to President Joe Biden, Republican Sens. Jodi Ernst and Ted Budd said they were "shocked" and "deeply concerned" about reports that the Biden administration failed to notify Congress before withholding ammunition to Israel.
"If these reports are true, then you have once again broken your promise to an American ally," they said.
According to the senior administration official, all of the shipments under review come from previously appropriated funds and are not sourced from the latest aid bill passed by Congress.
"We are committed to ensuring Israel gets every dollar appropriated in the supplemental," the official said, noting another $827 million in new future weapons and equipment for Israel has been approved through Foreign Military Financing.
Axios first reported on Sunday that a shipment of ammunition from the U.S. to Israel had been held up.
ABC News' Lauren Peller, Shannon K. Crawford, Matthew Seyler, Martha Raddatz, Benjamin Siegel and Molly Nagle contributed to this report.