SIMI VALLEY, California — Defense Secretary Lloyd Austin offered a stark public warning to Israel on Saturday, saying the government there risks “strategic defeat” in Gaza if it doesn’t do more to protect civilians.

The comments came a day after the breakdown in negotiations over hostage and prisoner releases led to the resumption of the Israel-Hamas war. Austin told an audience at the Reagan National Defense Forum that if Israel doesn’t do everything possible to protect civilians, it might help “drive [Palestinians] into the arms of the enemy,” and would “ replace a tactical victory with a strategic defeat.”

The Biden administration has been warning its Israeli counterparts in private over the human toll of Israeli airstrikes in Gaza, where Hamas has placed rocket launchers and command-and-control centers in civilian neighborhoods.

Austin said the U.S. learned hard lessons in Iraq and Afghanistan over killing and displacing civilians and acknowledged that fighting a war in a dense urban center places significant burdens on a democratic nation trying to follow the laws of war.

“The lesson is not that you can win in urban warfare by protecting civilians,” said Austin, who commanded troops in the Middle East. “The lesson is that you can only win in urban warfare by protecting civilians.”

Israel has repeatedly warned the people of Gaza to move from sites of active fighting and has dropped leaflets advising civilians to move south, away from some of the heaviest early fighting. Hamas killed 1,200 Israeli civilians in a cross-border attack Oct. 7 that focused on murdering and kidnapping civilians.

Israel’s renewed bombing has started to target areas in the south of Gaza, and Israel wanted Gazans away from some areas since Friday. The refugees from northern Gaza are increasingly being asked to move into smaller areas, exacerbating a humanitarian crisis across the strip.

Secretary of State Antony Blinken was in Israel and the U.A.E. late last week, and on Friday he blamed Hamas for the failure of the temporary cease-fire. It is “important to understand why the pause came to an end: It came to an end because of Hamas. Hamas reneged on commitments it made.”

Hamas fired rockets into Israel on Friday before the truce ended and failed to produce Israeli hostages it had pledged to release.

On Saturday, Israeli Defense Minister Yoav Gallant said that after releasing over 80 hostages, “Hamas violated the agreed framework” by refusing to release 17 more women and children.

“As a result of Hamas’ decision not to fulfill that which was agreed upon, and in accordance with the War Cabinet’s decision, yesterday morning I instructed the IDF to resume fire,” he said.