Israel-Gaza updates: Israeli forces preparing for 'wide range of offensive plans'

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.

All major U.S. airlines and a number of foreign carriers have suspended service to and from Israel.

Delta said it has suspended flights to and from Tel Aviv until the end of October. American Airlines has canceled flights to and from Tel Aviv through Oct. 13. United Airlines said its flights to and from Tel Aviv "will remain suspended until conditions allow them to resume."

Many other airlines, including Israeli flag carrier El Al, continue to operate flights.

The Federal Aviation Administration urged U.S. airlines and pilots to use caution when flying in Israeli airspace.

The European Union Aviation Safety Agency issued a Conflict Zone Information Bulletin for the airspace of Israel on Sunday, recommending air operators "ensure that a robust risk assessment is in place together with a high level of contingency planning for their operations and to be ready for short notice instructions from the Israeli authorities."

-ABC News' Amanda Maile