Fighting between Israel and Hamas resumed the day before after a week-long truce between the two sides collapsed, with both sides blaming the other for the breakdown of the deal and the resumption of violence.

Austin told the Reagan National Defense Forum in California that he had "learned a thing or two about urban warfare" while fighting in Iraq and leading the campaign against the Islamic State jihadist group (ISIS).

"Like Hamas, ISIS was deeply embedded in urban areas. And the international coalition against ISIS worked hard to protect civilians and create humanitarian corridors, even during the toughest battles," Austin said.

"The lesson is not that you can win in urban warfare by protecting civilians. The lesson is that you can only win in urban warfare by protecting civilians," he said.

"In this kind of a fight, the center of gravity is the civilian population. And if you drive them into the arms of the enemy, you replace a tactical victory with a strategic defeat."

The latest round of fighting in the long-running conflict between Israel and Hamas began when the Palestinian militant group carried out a shock cross-border attack from Gaza on October 7 that Israeli officials say killed about 1,200 people.

Israel responded with a relentless land and air campaign on Hamas-controlled Gaza that the group's officials say has killed more than 15,000 people.

Those deaths have provoked widespread anger in the Middle East and provided an impetus for armed groups to carry out attacks against American troops in the region as well as on Israel.

Israel has faced drone and missiles launched from Lebanon and Yemen, while American forces in Iraq and Syria have been targeted in a series of attacks that have injured dozens of US personnel.

Washington has blamed the attacks on its personnel on Iran-backed forces and responded with air strikes on multiple occasions in recent weeks.

"We will not tolerate attacks on American personnel. And so these attacks must stop," Austin said. "Until they do, we will do what we need to do to protect our troops -- and to impose costs on those who attack them."