WASHINGTON — The United States warned on Thursday that Israel will be dealing a strategic victory to Hamas if it carries out plans for an all-out assault on Rafah, the fighters’ last major stronghold in Gaza.

The warning was backed by a new threat from President Joe Biden: He says he will pause more offensive military assistance to Israel if it goes through with the operation in a city where more than 1 million civilians are sheltering.

Biden last week put on hold a shipment of large bombs to Israel over concerns the weapons are of the type that has caused significant civilian casualties in Gaza and would almost certainly do more such damage if Israel conducted a major offensive in Rafah.

On Wednesday, he held out the possibility of holding up future shipments of bomb guidance kits and artillery to Israel, in hopes the threat would turn Israel back from an operation in the city.

The pronouncements are part of last-ditch push for Israeli Prime Minister Benjamin Netanyahu and his far-right government to rethink their public commitments to invade the city in an effort to eradicate Hamas. The U.S. believes such a move would result in significant civilian casualties and exacerbate an already dire humanitarian crisis in Gaza.

The U.S. is making its sharpest moves yet to influence the decision-making of its ally in the ongoing war against the group that was triggered by Hamas’ Oct. 7 attack on Israel. Some 1,200 people in Israel were killed and about 250 were taken captive.

“Our view is any kind of major Rafah ground operation would actually strengthen Hamas’ hands at the negotiating table, not Israel’s,” White House national security spokesman John Kirby said Thursday. He said more civilian deaths in Rafah from an Israeli offensive would give more ammunition to Hamas’ “twisted narrative” about Israel.

Talks in Cairo aimed at securing a six-week cease-fire to allow for the release of some hostages and a surge of food and aid to civilians in Gaza are continuing, Kirby added. But CIA Director Bill Burns and other delegations to the talks left Egypt on Thursday without a deal.

Kirby said it was too soon to know whether the aid holdup had altered the Israeli calculus, but that the U.S. was continuing to advise Israel on how it could defeat Hamas through more surgical operations.

“We believe that they have put an enormous amount of pressure on Hamas and that there are better ways to go after what is left of Hamas in Rafah than a major ground operation,” he added.

Biden, in an interview with CNN on Wednesday, insisted that despite the arms hold up, the U.S. was still committed to Israel’s defense and would supply Iron Dome rocket interceptors and other defensive arms.

He acknowledged that “civilians have been killed in Gaza” by the type of heavy bombs that the U.S. has been supplying. It was his first validation of what administration critics have been loudly protesting, even if he still stopped short of taking responsibility. His threat to hold up artillery shells expanded on earlier revelations that the U.S. was going to pause a shipment of heavy bombs.

Biden said Israel’s actions around Rafah had “not yet” crossed his red lines, but he has repeated that Israel needs to do far more to protect the lives of civilians in Gaza. The Hamas-run health ministry puts the toll at more 34,000 dead, though it doesn’t distinguish between fighters and civilians.

Israel’s military spokesman, Rear Adm. Daniel Hagari, said that despite the American pause, “we have what we need” to press ahead with the mission. His comments came after Netanayahu brushed Biden’s threat, saying in a statement, “If we have to stand alone, we will stand alone.”

The U.S. has historically provided enormous amounts of military aid to Israel. The shipment that was paused was supposed to consist of 1,800 2,000-pound bombs and 1,700 500-pound bombs, according to a senior U.S. administration official who spoke on the condition of anonymity to discuss the sensitive matter. The focus of U.S. concern was the larger explosives and how they could be used in a dense urban area.

“I made it clear that if they go into Rafah — they haven’t gone in Rafah yet — if they go into Rafah, I’m not supplying the weapons that have been used historically to deal with Rafah, to deal with the cities, that deal with that problem,” Biden said.

“We’re not walking away from Israel’s security,” the Democratic president continued. “We’re walking away from Israel’s ability to wage war in those areas.”

U.S. officials had declined for days to comment on the halted transfer. Word about it came as Biden on Tuesday described U.S. support for Israel as “ironclad, even when we disagree.”

Israel’s ambassador to the United Nations, Gilad Erdan, in an interview with Israeli Channel 12 TV news, said the decision to pause the shipment was “a very disappointing decision, even frustrating.” He suggested the move stemmed from political pressure on Biden from Congress, the U.S. campus protests and the upcoming election.

The decision also drew a sharp rebuke from House Speaker Mike Johnson, R-La., and Senate Republican leader Mitch McConnell of Kentucky, who said they only learned about the holdup from press reports, despite assurances from the Biden administration that no such pauses were in the works. The Republicans called on Biden in a letter to swiftly end the blockage, saying it “risks emboldening Israel’s enemies,” and to brief lawmakers on the nature of the policy reviews.

Biden has faced pressure from some on the left and condemnation from the critics on the right who say Biden has moderated his support for an essential Mideast ally.

“The American people support Israel overwhelmingly,” said South Dakota Sen. John Thune, the No. 2 Republican, who pushed a resolution condemning Biden’s decision. “And they also believe that Israel needs to do what is necessary, and if that includes going into Rafah to root out the Hamas threat, then that is necessary for their very survival.”

Former President Donald Trump, entering a New York courthouse for his criminal trial over hush money payments, criticized Biden as well, saying Thursday that “What Biden is doing with respect to Israel is disgraceful.” The presumptive GOP presidential nominee added, “If any Jewish person voted for Joe Biden, they should be ashamed of themselves. He’s totally abandoned Israel.”

Independent Sen. Bernie Sanders of Vermont, a Biden ally, said in a statement the pause on big bombs must be a “first step.”

“Our leverage is clear,” Sanders said. “Over the years, the United States has provided tens of billions of dollars in military aid to Israel. We can no longer be complicit in Netanyahu’s horrific war against the Palestinian people.”

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.

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.

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.

Associated Press writers Josef Federman in Jerusalem and Lolita C. Baldor and Matthew Lee contributed to this report.