"I made it clear that if they go into Rafah, ... I’m not supplying the weapons," Biden, whose administration has repeatedly asked Israel for its plan to protect civilians in Rafah, told CNN in an interview.
Biden acknowledged that U.S. bombs provided to Israel have killed Gaza civilians in the seven-month-old offensive aimed at annihilating Hamas.
The comments, Biden's starkest yet, increase the pressure on Israel to refrain from a full-scale assault on Rafah, where hundreds of thousands of Palestinians have sought refuge after fleeing combat farther north in Gaza.
There was no immediate comment from Israel on Biden's remarks, but Israeli Prime Minister Benjamin Netanyahu has said the Rafah operation would go ahead. Israel says it must hit Rafah to defeat thousands of Hamas fighters it says are there.
Israel kept up tank and aerial strikes on southern Gaza after moving in via the Rafah border crossing with Egypt on Tuesday, cutting off a vital aid route.
Biden has been under pressure from his fellow Democrats and growing campus protests to deter Israel from invading Rafah. His support of Israel has become a political liability as the president runs for re-election.
Biden said U.S. weapons for Israel's defense, such as for its Iron Dome anti-missile system, would continue.
On Wednesday, U.S. officials confirmed that Washington paused delivery of an arms shipment of 1,800 2,000-pound (907-kg) bombs and 1,700 500-pound bombs to Israel because of the risk to civilians in Gaza.
Four sources said the shipments, delayed for at least two weeks, involved Boeing-made Joint Direct Attack Munitions, which convert dumb bombs into precision-guided ones, as well as Small Diameter Bombs (SDB-1).
The United States is by far the biggest supplier of weapons to Israel, and it accelerated deliveries after Oct 7.
They were part of an earlier approved shipment to Israel, not the recent $95-billion supplemental aid package the U.S. Congress passed in April.
Israel's U.N. ambassador, Gilad Erdan, called that decision "very disappointing" but said he did not believe the U.S. would stop supplying arms to Israel.
TALKS IN CAIRO
Palestinian group Hamas said late on Wednesday it would not make more concessions to Israel in the truce talks.
In Cairo, delegations from Hamas, Israel, the U.S., Egypt and Qatar have been meeting since Tuesday.
Citing a source familiar with the matter, Egypt's state-affiliated Al Qahera TV said early on Thursday that areas of disagreement were being resolved and there were signs an agreement will be reached, without giving details.
But Izzat El-Reshiq, a member of Hamas' political office in Qatar, said in a statement late on Wednesday the group would not go beyond a ceasefire proposal it accepted on Monday.
That would also entail the release of some Israeli hostages in Gaza and Palestinian women and children detained in Israel.
"Israel isn’t serious about reaching an agreement and it is using the negotiation as a cover to invade Rafah and occupy the crossing," said Reshiq.
On Monday Israel declared that the three-phase truce proposal approved by Hamas was unacceptable because terms had been watered down. It did not respond immediately to the Hamas statement.
The U.S. said on Tuesday that Hamas had revised its ceasefire proposal and the revision could overcome an impasse in negotiations. Just a few hours before Hamas' latest statement, Washington continued to say the two sides were not far apart.
"We believe there is a pathway to a deal ... The two sides are close enough they should do what they can to get to a deal," U.S. national security adviser Jake Sullivan told reporters.
'HUMANITARIAN CATASTROPHE' LOOMS
Hamas said its fighters on Wednesday were battling Israeli forces in Rafah's east and Islamic Jihad's fighters attacked Israeli soldiers and military vehicles with heavy artillery near the city's long abandoned airport.
Israeli tank shells landed in the middle of Rafah, wounding at least 25 people on Wednesday, medics said. Residents said an Israeli air strike killed four people and wounded 16 in western Rafah.
The Israeli military said its troops had discovered Hamas infrastructure in several places in eastern Rafah and were conducting targeted raids in Rafah and airstrikes across the Gaza Strip.
The U.N., Gaza residents and humanitarian groups say further Israeli incursion into Rafah will result in a humanitarian catastrophe.
A U.N. official said no fuel or aid had entered the Gaza Strip due to the military operation, a situation "disastrous for the humanitarian response" in Gaza where more than half the population is suffering catastrophic hunger.
Palestinians have crammed into tented camps and makeshift shelters, battling shortages of food, water and medicine.
