The White House warned Israel against carrying out a ground invasion of Rafah, saying that it would not serve its objectives of defeating Hamas in Gaza.

"Smashing into Rafah, in his view, will not advance that objective," White House national security spokesman John Kirby said in a briefing with reporters.

Kirby said Hamas had been pressured significantly by Israel and that there were better ways now to go after what remains of the group's leadership than an operation with significant risk to civilians.

"The argument that somehow we're walking away from Israel, or we're not willing to help them defeat Hamas just doesn't comport with the facts."

The current round of negotiations in Cairo over a possible hostage-release deal have ended without apparent resolution of differences, a senior Israeli official said on Thursday. Israel will proceed with its operation in Rafah as planned, the official added.

"We still believe that there's a path forward, but it's gonna take some leadership on both sides," said Kirby. "And it's going to take a little bit of moral courage on both sides to finally be able to come across that table and ink this deal."

Kirby said U.S.-Israel consultations were ongoing about alternatives to a major ground operation in Rafah.

"That's a choice that Israel will have to make," he said of the offensive in densely packed Rafah. "And it's one we hope they don't."

Also on Thursday, President Joe Biden said that he would not supply offensive weapons that Israel could use to launch an all-out assault on Rafah - the last major Hamas stronghold in Gaza - over concern for the well-being of the more than 1 million civilians sheltering there.

Biden, in an interview with CNN on Wednesday, said that the U.S. was still committed to Israel's defense and would supply Iron Dome rocket interceptors and other defensive arms but that if Israel goes into Rafah, "we’re not going to supply the weapons and artillery shells used."

Biden acknowledged that "civilians have been killed in Gaza" by the type of heavy bombs that the U.S. has been supplying -- 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.

The U.S. has historically provided enormous amounts of military aid to Israel. Biden's comments and his decision last week to pause the shipment of heavy bombs to Israel are the most striking manifestations of the growing daylight between his administration and Israel Prime Minister Benjamin Netanyahu’s government. Biden said Wednesday that Israel’s actions around Rafah had "not yet" crossed his red lines, but has repeated that Israel needs to do far more to protect the lives of civilians in Gaza.

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 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.

"Civilians have been killed in Gaza as a consequence of those bombs and other ways in which they go after population centers," Biden told CNN. "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."

"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."

Defense Secretary Lloyd Austin earlier Wednesday confirmed the weapons delay, telling the Senate Appropriations Subcommittee on Defense 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."

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.