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

A 74-year-old mother who is missing in Israel fought for peace her whole life, her son told ABC News Live on Monday.

Yonatan Zeigen said he last heard from his mother, Vivian Silver, at 11:07 a.m. on Saturday morning while she was hiding in a closet as Hamas fighters entered her house.

Zeigen added that he didn't know if Israeli soldiers had been to her house yet and hadn't heard "anything concrete" about the search yet.

Zeigen talked about his mother's commitment to peace, since Silver moved to a kibbutz in Israel in the 1990s.

"She fought for equality," he said. "She's been involved with a lot of different organizations promoting peace and promoting solution for the conflict.

"Up until recently, she would drive sick Palestinians from the Gaza border to Israeli hospitals through an organization called Road to Recovery," Zeigen said.

"No, I'm primarily sad, and anxious," Zeigen said when asked if he was angry about the situation. "War is blind. You can do what you do in your life...it doesn't make you any less of a target in times of war."

-ABC News' Michelle Stoddart

Former President Barack Obama released a statement Monday evening condemning Hamas' attack on Israel over the weekend.

"All Americans should be horrified and outraged by the brazen terrorist attacks on Israel and the slaughter of innocent civilians," Obama said in his statement. "We grieve for those who died, pray for the safe return of those who’ve been held hostage, and stand squarely alongside our ally, Israel, as it dismantles Hamas."

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