Israel-Gaza-Lebanon live updates: Israel says it's now striking Iran

Israel’s planned attack on Iran will mark the "beginning of a war" that will last "many months," retired Israel Defense Forces Brig. Gen. Amir Avivi told ABC News.

Israel's war with Hezbollah had gone incredibly well and the IDF had surpassed its own expectations in degrading the Iranian proxy, Avivi said, adding that the same mentality would be applied to dealing with the Iranian regime itself.

"We have a historical opportunity to deal with Iran so they don’t pose another threat to Israel," said Avivi, the founder of the right-wing think tank Israel's Defense and Security Forum.

Israel's retaliatory strike on Iran will be "fierce and strong and very surprising" when it happens, he said.

-ABC News' Tom Soufi Burridge and Jordana Miller

The Israel Defense Forces said it hit 300 Hezbollah targets in southern Lebanon in the past 24 hours, which saw an intense night of airstrikes on Beirut.

Seven brigade commanders, 21 battalion commanders and 24 company commanders for Hezbollah were killed in its ongoing operations, the IDF said.

More strikes against Hezbollah targets are expected all over southern Lebanon Monday night, IDF spokesperson Rear Adm. Daniel Hagari said.

There's no indication yet whether classified documents on Israel's retaliation plans were leaked or hacked, White House national security spokesperson John Kirby told reporters Monday.

"I'm just not able to answer your question whether it was a leak or a hack at this point. We'll let the investigation pursue its logical course there," Kirby said.

Kirby said that President Joe Biden was "deeply concerned" about the incident, and that while they don't expect more documents to be revealed, they are on high alert amid the investigation.

"We're certainly going to keep our antenna up and our eyes open for any potential future disclosures," he said.

-ABC News' Cheyenne Haslett

Seven Israeli citizens were arrested after allegedly spying for Iran, Israeli authorities said Monday.

The Israel Security Agency and Israel Police said they "successfully dismantled a spy network" that allegedly gathered sensitive information on Israel Defense Forces bases and energy infrastructure.

The citizens were allegedly recruited by Iranian agents to conduct "security-related tasks" over at least two years, authorities said, including "extensive reconnaissance missions" on air force and navy installations, ports, Iron Dome system locations and energy infrastructure.

Photographs and videos of "numerous" IDF bases, ports and energy infrastructure were seized as part of the investigation, authorities said.

"It is assessed that these activities have inflicted security damage on the state," the ISA and Israel Police said.

The seven Israelis were allegedly paid hundreds of thousands of dollars, often through cryptocurrencies, for their work, authorities said.

Prosecutors are expected to file an indictment against them in the coming days.