The United States will stop supplying offensive weaponry to the Israelis if the country goes through with a planned invasion of the city of Rafah in the Gaza Strip, President Joe Biden said on Wednesday, a signal that he is willing to put conditions on aid to Israel for the first time and that he’s reached a major break with Prime Minister Benjamin Netanyahu.

“Civilians have been killed in Gaza as a consequence of those bombs and other ways in which they go after population centers,” Biden said during an interview with CNN, acknowledging the deaths of tens of thousands of Palestinian civilians during the war launched in retaliation for Hamas’ massacre of Israelis and taking of hostages on Oct. 7.

“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,” Biden said.

Biden, long a staunch and unquestioning supporter of Israel, has come under intense pressure from Democrats to finally place conditions on military aid to the country, which has been accused of war crimes and has been deemed at risk of committing genocide in its attacks on the Palestinian enclave of Gaza. Biden said the U.S. would continue to provide for Israel’s defense, as it did when it assisted the country against an attack from Iran last month.

“We’re going to continue to make sure Israel is secure in terms of Iron Dome and their ability to respond to attacks that came out of the Middle East recently,” he said, adding, “We’re not going to supply the weapons and artillery shells.”

Rafah, where 1 million displaced Palestinians are sheltering, is one of the last remaining safe zones in the Gaza Strip. Humanitarian groups fear a full-on Israeli invasion of the city would result in mass civilian deaths.

“I’ve made it clear to Bibi [Netanyahu] and the war cabinet: They’re not going to get our support if in fact they go on these population centers,” said Biden, who spoke to Netanyahu on Monday.

Israel has characterized the southern Gaza city as the militant group Hamas’ last stronghold. Israeli tanks moved into part of Rafah on Tuesday, seizing the border crossing between Gaza and Egypt. Israeli officials described it as a limited operation in response to a Hamas rocket attack that killed four Israeli troops just outside Gaza on Sunday.

But the Israeli incursion and an evacuation order to 100,000 Rafah residents have prompted a stricter line from the Biden administration, which fears the consequences of a broader Israeli invasion of the city.

News of delayed weapon shipments to Israel first arrived this weekend, and Secretary of Defense Lloyd Austin confirmed the reports at a congressional hearing Wednesday. So far, the administration has delayed transferring 2,000-pound and 500-pound explosives.

Austin faced his share of tough questions and comments from Republican senators about the decision.

“This is obscene. It is absurd,” Sen. Lindsey Graham (R-S.C.) remarked during the hearing. “Give Israel what they need to fight the war. They can’t afford to lose. This is Hiroshima and Nagasaki on steroids.”

Biden worked closely with Senate Republicans and House Speaker Mike Johnson (R-La.) to pass a $95 billion supplemental foreign aid bill in late April that includes $26.4 billion in aid to Israel and $1 billion for humanitarian assistance in Gaza.

The White House has assured Republican leaders that none of the delayed weapon shipments are part of the assistance allotted in the supplemental bill.

But Johnson and Senate GOP Leader Mitch McConnell (R-Ky.) sent a joint letter to Biden on Wednesday expressing discontent about hearing of the arms delay first in the press, and about the nature of the delay itself.

“We believe that security assistance to Israel is an urgent priority that must not be delayed,” McConnell and Johnson wrote.

Biden’s decision to use U.S. military aid as leverage to shape Israeli policy is the culmination of months of tension building with Netanyahu. The administration has gone from insisting that Israel present a realistic plan for ensuring the safety of civilians in Rafah in the event that they invade the city to arguing more openly that it sees a full-scale invasion as inherently incompatible with its humanitarian concerns.

The dust-up over Rafah also takes place against the backdrop of hostage negotiations between Israel and Hamas in which the U.S., Egypt and Qatar have played active roles. Israel this week rejected a counter-offer from Hamas but has said it would keep negotiating.

Domestic pressure is mounting on Netanyahu to strike a deal that secures the return of as many hostages held by Hamas as possible. Family members of hostages were among the thousands of Israelis who marched in Tel Aviv on Saturday demanding that Netanyahu reach an agreement.

At the same time, Netanyahu’s coalition government depends on the support of far-right parties led by Finance Minister Bezalel Smotrich and National Security Minister Itamar Ben-Gvir. Those parties have threatened to collapse Netanyahu’s government if he does not invade Rafah.

In his remarks at a Holocaust remembrance event on Sunday, Netanyahu suggested that he is prepared to defy the United States.

“If Israel is forced to stand alone, Israel will stand alone,” he declared.