President Joe Biden today publicly warned Israel for the first time that the U.S. would stop supplying it weapons if Israeli forces make a major invasion of Rafah, a refugee-packed city in southern Gaza.

“I made it clear that 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 in an interview with CNN.

Biden’s comments represent his strongest public language to date in his effort to deter an Israeli assault on Rafah while underscoring a growing rift between the U.S. and its strongest ally in the Middle East.

Biden acknowledged U.S. weapons have been used by Israel to kill civilians in Gaza, where Israel has mounted a seven-month-old offensive aimed at annihilating Hamas. Israel’s campaign has so far killed 34,789 Palestinians, mostly civilians, the Gaza Health Ministry said.

“Civilians have been killed in Gaza as a consequence of those bombs and other ways in which they go after population centers,” he said when asked about 2,000-pound bombs sent to Israel.

Israel this week attacked Rafah, where more than one million Palestinians have sought refuge, but Biden said he did not consider Israel’s strikes a full-scale invasion because they have not struck “population centers.”

A senior U.S. official, speaking on condition of anonymity, said Washington had carefully reviewed the delivery of weapons that might be used in Rafah and as a result paused a shipment consisting of 1,800 2,000-pound (907-kg) bombs and 1,700 500-pound bombs.

The interview was released hours after Defense Secretary Lloyd J. Austin III acknowledged publicly Biden’s decision last week to hold up the delivery of thousands of heavy bombs was taken out of concern for Rafah, where Washington opposes a major Israeli invasion without civilian safeguards.

Israel’s campaign in Gaza was triggered by Hamas ’ Oct. 7 attack on Israel. That killed about 1,200 people with about 250 others abducted, of whom 133 are believed to remain in captivity in Gaza, according to Israeli tallies.

Biden said the U.S. would continue to provide defensive weapons to Israel, including for its Iron Dome air defense system.

“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. “But it’s, it’s just wrong. We’re not going to – we’re not going to supply the weapons and artillery shells.”