WASHINGTON (AP) — The U.S. paused a shipment of bombs to Israel last week over concerns that the country was approaching a decision on launching a full-scale assault on the southern Gaza city of Rafah against the wishes of the U.S., Defense Secretary Lloyd Austin said Wednesday.

The shipment was supposed to consist of 1,800 2,000-pound (900-kilogram) bombs and 1,700 500-pound (225-kilogram) bombs, according to the official who spoke on the condition of anonymity. The focus of U.S. concern was the larger explosives and how they could be used in a dense urban setting like Rafa where more than 1 million civilians are sheltering after evacuating other parts of Gaza amid Israel's war on Hamas, which came after the militant group's deadly attack on Israel on Oct. 7.

Austin confirmed the weapons delay, telling the Senate Appropriations Defense subcommittee that the U.S. paused "one shipment of high payload munitions."

"We're going to continue to do what's necessary to ensure that Israel has the means to defend itself," Austin said. "But that said, we are currently reviewing some near-term security assistance shipments in the context of unfolding events in Rafah."

The U.S. has historically provided enormous amounts of military aid to Israel. That has only accelerated in the aftermath of Hamas' Oct. 7 attack that killed some 1,200 in Israel and led to about 250 being taken captive by militants. The pausing of the aid shipment is the most striking manifestation of the growing daylight between Israel Prime Minister Benjamin Netanyahu's government and the administration of President Joe Biden, which has called on Israel to do far more to protect the lives of innocent civilians in Gaza.

It also comes as the Biden administration is due to deliver a first-of-its-kind formal verdict this week on whether the airstrikes on Gaza and restrictions on delivery of aid have violated international and U.S. laws designed to spare civilians from the worst horrors of war. A decision against Israel would further add to pressure on Biden to curb the flow of weapons and money to Israel's military.

Biden signed off on the pause in an order conveyed last week to the Pentagon, according to U.S. officials who were not authorized to comment on the matter. The White House National Security Council sought to keep the decision out of the public eye for several days until it had a better understanding of the scope of Israel's intensified military operations in Rafah and until Biden could deliver a long-planned speech on Tuesday to mark Holocaust Remembrance Day.

Biden's administration in April began reviewing future transfers of military assistance as Netanyahu's government appeared to move closer toward an invasion of Rafah, despite months of opposition from the White House. The official said the decision to pause the shipment was made last week and no final decision had been made yet on whether to proceed with the shipment at a later date.

U.S. officials had declined for days to comment on the halted transfer, word of which came as Biden on Tuesday described U.S. support for Israel as "ironclad, even when we disagree."

Press secretary Karine Jean-Pierre declined to square the arms holdup with Biden's rhetoric in support of Israel, saying only, "Two things could be true."

Israeli troops on Tuesday seized control of Gaza's vital Rafah border crossing in what the White House described as a limited operation that stopped short of the full-on Israeli invasion of the city that Biden has repeatedly warned against on humanitarian grounds, most recently in a Monday call with Netanyahu.

Israel has ordered the evacuation of 100,000 Palestinians from the city. Israeli forces have also carried out what it describes as "targeted strikes" on the eastern part of Rafah and captured the Rafah crossing, a critical conduit for the flow of humanitarian aid along the Gaza-Egypt border.

Privately, concern has mounted inside the White House about what's unfolding in Rafah, but publicly administration officials have stressed that they did not think the operations had defied Biden's warnings against a widescale operation in the city.

White House National Security Council spokesman John Kirby said Israel described the operation along the Gaza-Egypt border in eastern Rafah as "an operation of limited scale and duration" aimed at cutting off Hamas arms smuggling, but also said the U.S. would monitor the fighting.

Just last month, Congress passed a $95 billion national security bill that included funding for Ukraine, Israel and other allies. The package included more than $14 billion in military aid for Israel, though the stalled transfer was not related to that measure.

The State Department is separately considering whether to approve the continued transfer of Joint Direct Attack Munition kits, which place precision guidance systems onto bombs, to Israel, but the review didn't pertain to imminent shipments.

The U.S. dropped the 2,000-pound bomb sparingly in its long war against the Islamic State militant group. Israel, by contrast, has used the bomb frequently in the seven-month Gaza war. Experts say the use of the weapon, in part, has helped drive the enormous Palestinian casualty count that the Hamas-run health ministry puts at more than 34,000 dead, though it doesn't distinguish between militants and civilians.

The U.S.-Israel relationship has been close through both Democratic and Republican administrations. But there have been other moments of deep tension since Israel's founding in which U.S. leaders have threatened to hold up aid in attempt to sway Israeli leadership.

President Dwight Eisenhower pressured Israel with the threat of sanctions into withdrawing from the Sinai in 1957 in the midst of the Suez Crisis. Ronald Reagan delayed the delivery of F16 fighter jets to Israel at a time of escalating violence in the Middle East. President George H.W. Bush held up $10 billion in loan guarantees to force the cessation of Israeli settlement activity in the occupied territories.

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AP writers Lolita C. Baldor and Matthew Lee contributed to this report.