US President Joe Biden has warned Israel that the US will stop supplying some weapons if it launches a major ground operation in Rafah.
Mr Biden, in an interview with CNN, said the US was still committed to Israel's defence and would supply Iron Dome rocket interceptors and other defensive arms, but that if Israel goes into the city in Gaza "we're not going to supply the weapons and artillery shells used, that have been used".
The interview marked Mr Biden's toughest public comments yet about the potential Israeli military operation.
It follows his decision to pause a shipment of heavy bombs to Israel last week over concerns that the US ally was moving closer to an attack on Rafah despite public and private warnings from his administration.
Rafah is considered Hamas's last major stronghold in Gaza but more than a million people are sheltering there after Israel’s military operation in other parts of the Palestinian territory.
US officials have warned that an operation in the city - where the population has swelled with refugees from other parts of Gaza - could lead to extensive civilian casualties.
The US has historically provided enormous amounts of military aid to Israel.
That has only accelerated in the aftermath of Hamas' October 7 attack that killed some 1,200 in Israel and led to about 250 being taken captive by militants.
Mr Biden's comments and the pausing of a 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.
Mr Biden has said that Israel needs to do far more to protect the lives of civilians in Gaza.
The shipment was supposed to consist of 1,800 2,000lb bombs and 1,700 500lb bombs, according to a senior US administration official.
The focus of US concern was the larger explosives and how they could be used in a dense urban setting.
"Civilians have been killed in Gaza as a consequence of those bombs and other ways in which they go after population centres," Mr 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."
Defence secretary Lloyd Austin earlier on Wednesday confirmed the weapons delay, telling the Senate appropriations subcommittee on defence that the US 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," Mr 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 US laws designed to spare civilians from the worst horrors of war.
A decision against Israel would further add to pressure on Mr Biden to curb the flow of weapons and money to Israel's military.
Mr Biden signed off on the pause in an order conveyed last week to the Pentagon, according to US officials.
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 Mr Biden could deliver a long-planned speech on Tuesday to mark Holocaust Remembrance Day.
Mr Biden's administration in April began reviewing future transfers of military assistance as Mr 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.
US officials had declined for days to comment on the halted transfer, word of which came as Mr Biden on Tuesday described US 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 Mr Biden from Congress, the US campus protests and the upcoming election.
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 Mr Biden has repeatedly warned against on humanitarian grounds, most recently in a Monday call with Mr 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.
