The Israeli Defense Forces said it "struck terror targets" belonging to Hamas in Gaza.

"IDF aircraft struck a Hamas operational command center in a mosque and an offensive tunnel entry point that was used by terrorists to invade Israel," the IDF said in a statement.

The IDF also claimed it struck an "Islamic Jihad operational meeting point inside a house that was occupied by a large number of terrorists."

"Two terrorists were killed when they attempted to flee the scene," the IDF said.

-ABC News' Will Gretsky

The State Department confirmed Monday evening that there are still missing Americans in Israel but couldn't immediately provide an exact number or estimate.

The department couldn't say yet if any of the missing Americans were hostages.

White House spokesperson John Kirby echoed this sentiment to ABC News' Linsey Davis.

"We don't really know whether they're just missing somewhere, or are lost, or whether they're being held hostage. I think we have to accept the possibility that at least some of them are being held hostage by Hamas," he said.

Of the Americans previously confirmed dead, a State Department official said they were all thought to be dual U.S.-Israeli citizens primarily residing in Israel.

Staffers at the U.S. Embassy in Israel are fully accounted for and are communicating with impacted Americans around the clock over the telephone and via the online form the department launched for U.S. nationals who believe their family members are among those missing, according to the agency.

The State Department is also urging Americans in the affected areas who are safe to contact their loved ones directly and/or update their status on social media.

-ABC News' Shannon Crawford

At least 687 people have died, including at least 140 children and 105 women, from Israeli airstrikes in the Gaza Strip since Saturday, according to Palestinian officials.

The Israel Defense Forces said over 1,500 military targets were hit in Gaza on Monday.

The death toll has continued to grow on both sides as Israeli forces strike back at Gaza's militant Hamas rulers.

At least 11 Americans are among those killed in Israel, President Joe Biden said in a statement.

He added, "While we are still working to confirm, we believe it is likely that American citizens may be among those being held by Hamas."

"American citizens still remain unaccounted for, and we are working with Israeli officials to obtain more information as to their whereabouts," Biden said. "My heart goes out to every family impacted by the horrible events of the past few days. The pain these families have endured, the enormity of their loss, and the agony of those still awaiting information is unfathomable."

"I have directed my team to work with their Israeli counterparts on every aspect of the hostage crisis, including sharing intelligence and deploying experts from across the United States government to consult with and advise Israeli counterparts on hostage recovery efforts," Biden said.

In his first on-camera comments about Israel’s evacuation request for northern Gaza, Israeli Defense Minister Yoav Gallant urged Gaza residents to head south. He noted that Hamas will use them as "camouflage," “therefore we need to separate them."

"Those who want to save their life, please go south. We are going to destroy Hamas infrastructures, Hamas headquarters, Hamas military," he said.

Defense Secretary Lloyd Austin, who appeared at the press conference with Gallant, dodged ABC News' question about how Israel could end the Hamas terror threat without inevitably risking civilian lives. Austin called Israel's military "professional" and "disciplined."

Austin described how ISIS had also embedded themselves in civilian populations and how the anti-ISIS coalition "protected civilians and created corridors for humanitarian movement even in the midst of a pretty significant fight. So again, this is a professional force. It is well led, and I have every expectation that it will be disciplined."

Austin added, "I would tell you that in countering ISIS, I felt as if we were staring evil in the eye, it was truly evil. And what we've seen from Hamas, it takes that evil to another level."

-ABC News' Matt Seyler and Luis Martinez