President Biden acknowledged on Wednesday that American bombs have been used to kill Palestinian civilians as he warned that the United States would withhold certain weapons if Israel launches a long-threatened assault in southern Gaza.

In some of his strongest language to date on the seven-month war, Mr. Biden said the United States would still ensure Israel’s security, including the Iron Dome missile defense system and Israel’s “ability to respond to attacks” like the one Iran launched in April.

But he said he would block the delivery of weapons that could be fired into densely populated areas of Rafah, where more than a million Palestinians are sheltering.

The president had already halted the shipment of 3,500 bombs last week out of concern that they might be used in a major assault on Rafah — the first time since Hamas attacked Israel on Oct. 7 that Mr. Biden has leveraged U.S. arms to try to influence how the war is waged.

On Wednesday, he said that he would also block the delivery of artillery shells.

“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,” Mr. Biden said in an interview with CNN’s Erin Burnett.

He added: “But it’s just wrong. We’re not going to — we’re not going to supply the weapons and artillery shells used, that have been used.”