Israel-Gaza live updates: Israel 'deepening the war' in Gaza: Netanyahu

Secretary of Defense Lloyd Austin said while in Israel he's offered "thoughts about how to transition from high intensity operations to a lower intensity and more surgical operations," and Austin said he and the Israelis have "had great discussions on all of those issues."

"In any operation like this, any campaign, there will be phases, and the most difficult part is as you shift from one phase to the next, making sure that you have everything accounted for and you get it right. So that requires detailed planning and very thoughtful planning," Austin said during a joint press conference in Israel Monday with Israeli Defense Minister Yoav Gallant.

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Defense secretary speaks out on Israel-Hamas war

ABC News’ Britt Clennett and Selina Wang break down the defense secretary’s comments on the Israel-Hamas war during a visit to Israel.

ABCNews.com

Gallant said their discussions have been "transparent and frank."

"On the personal level, I have a lot to learn from Gen. Austin," Gallant said. "He always gives me good advice."

Austin also reaffirmed America’s "unshakeable" support for Israel.

"At a time of mourning, a real friend shows up," Austin said. "And I know how terrible these days have been for the Israeli people. And I know that Oct. 7 touched everyone in this small democracy."

Austin emphasized the importance of limiting harm to civilians in Gaza, but he also stressed the "complexity" of the "battle space" in Gaza.

“Very closed spaces, dense population of people -- and so that makes it very, very difficult to conduct any military operation,” he said. "Above and beyond that, we see that Hamas routinely uses civilians as shields. Beyond that, they place their headquarters and their logistical sites near protected sites: hospitals, mosques, churches, you name it."

-ABC News’ Matt Seyler

Twenty-four aid trucks crossed through Israel's Kerem Shalom border crossing point into Gaza on Sunday, marking the first time aid had gone through this crossing since the war began, Palestinian Rafah border crossing spokesman Wael Abu Omar told ABC News.

Another 150 aid trucks crossed through the Egypt-Gaza Rafah border crossing on Sunday, the spokesman said.

The Kerem Shalom border crossing point closed after Hamas’ Oct. 7 attack on Israel. Last week, Israeli officials announced Kerem Shalom would open for security checks of aid trucks, but the trucks would still have to enter Gaza through the Rafah crossing. Then on Friday, Israel and the U.S. announced aid trucks would also be allowed to cross into Gaza from the Kerem Shalom crossing point.

-ABC News’ Ayat Al-Tawy

Gen. C.Q. Brown, the chairman of the Joint Chiefs of Staff, said that in meetings in Israel on Monday with senior Israeli political and military leaders, he and Defense Secretary Lloyd Austin will discuss Israel’s criteria for deciding when to transition to stability operations in Gaza at the end of its major military operations and ensuring it is "a smooth transition."

"The goal here is to have better security than they had prior to the events on 7 October," Brown told reporters Sunday while en route to Israel.

Austin and Brown speak regularly with their Israeli counterparts about the conflict with Hamas and urge Israel to protect civilian lives as it carries out its operations, but their trip will build on National Security Advisor Jake Sullivan’s meetings last week in Israel that had a similar focus.

"Part of this is us sharing our own experience and not building a plan for the Israelis, but actually talking through our own experiences and how we've gone through various conflicts in the past," explained Brown.

Brown said the next phase of stability operations in Gaza has to provide
"some level of stability to provide support for security, governance," and the continued flow of humanitarian aid.

He added that senior U.S. military officers can offer political leaders an idea of what will be needed to carry out those stability operations and ensure a smooth transition.

But Brown said he would also be listening to Israel's viewpoint.

"It's more of a dialogue that goes both ways, and I'm trying to understand where they're coming from and what they’re -- how they see things," he said. "And I try to share based on my experience and the things that we see on how best to move forward."

-ABC News’ Luis Martinez

Israel Defense Forces claimed on Sunday that it has discovered one of Hamas' biggest tunnels under the Gaza Strip and that it plans to destroy it.

Measuring nearly 2 1/2 miles long and large enough to drive a vehicle through, the tunnel's entrance was located roughly 1,300 feet from the Erez border crossing between Gaza and Israel, the IDF said in a post on the social media site X.

The shaft, constructed with reinforced concrete and reaching a depth of more than 160 feet, is believed to have taken years to build and millions of dollars to complete, according to the IDF. The tunnel is also equipped with electricity, communications networks and rails, according to the IDF.

It's unclear when the tunnel was found. The IDF said it also found weapons, militants and booby traps in the tunnel.

The tunnel, according to the IDF, is the brainchild of Mohammed Sinwar, the brother of Hamas leader Yahya Sinwar. The IDF released a Hamas video it seized showing Mohammed Sinwar in the passenger seat of a Jeep driving inside the tunnel.

IDF spokesperson Daniel Hagari told reporters that the tunnel would be destroyed.

"Without demolishing the tunnel project of Hamas, we cannot demolish Hamas," Hagari said.

The IDF claims to have destroyed more than 800 tunnels in Gaza since the beginning of the war.

ABC News' Ines De La Cuetara