At least three Americans killed in attacks in Israel near Gaza border, according to memo

At least four Americans have been killed in the attacks in Israel, and US officials expect the death toll to rise, Sen. Chuck Schumer said Sunday.

The Senate majority leader said in a statement that he was briefed Sunday evening by senior national security and State Department officials.

"Unfortunately, we know there are Americans who were killed. The Administration told us that they know of four thus far, but, sadly, we know the toll will rise," Schumer, a Democrat, said in a statement.

Earlier Sunday, Secretary of State Antony Blinken said the US was "working overtime" to verify reports of missing and dead Americans after Hamas launched an unprecedented attack against Israel.

"We have reports that several Americans were killed. We're working overtime to verify that. At the same time, there are reports of missing Americans and there again, we're working to verify those reports," Blinken told CNN's Dana Bash on "State of the Union."

CNN reported earlier Sunday that at least three Americans were killed, according to an internal US government memo. The memo said the State Department was aware of additional Americans who have not been accounted for.

CNN has reached out to the State Department for comment.

Israeli Minister of Strategic Affairs Ron Dermer told Bash Sunday that Americans are among the "scores" of hostages being held in Gaza.

The US Embassy in Jerusalem, meanwhile, updated its contingency planning "for any potential evacuation by land or air," according to the memo. On Saturday, the embassy issued a "shelter in place" order for its personnel.

The large-scale surprise attack by Gaza militants Saturday has left hundreds of Israelis dead, prompting vows of retribution from Israeli Prime Minister Benjamin Netanyahu and a lethal volley of retaliatory Israeli airstrikes.

It's unclear what Israel's full response will look like. An Israeli military spokesperson hinted that Israel may try to take full control of Gaza for the first time since 2005, and more than 20 communities near Gaza are being evacuated.

The top US diplomat added that President Joe Biden's direction was to "make sure that we're providing Israel everything it needs in this moment to deal with the attacks from Hamas."

Speaking from the White House on Saturday, Biden said his administration's support of Israel's security is "rock solid and unwavering."

In a Sunday conversation with Netanyahu, Biden said that assistance was "on its way," with additional help forthcoming.

Defense Secretary Lloyd Austin announced Sunday that a US carrier strike group was headed to the eastern Mediterranean Sea as a "deterrence posture." In his statement, Austin also said the US "will be rapidly providing the Israel Defense Forces with additional equipment and resources."

Still, the president and his top aides are contending with a complicated diplomatic situation unlike any previous conflict between the Israelis and the Palestinians.

Republican presidential candidates have slammed the Biden administration, with former President Donald Trump - the front-runner for the GOP nomination - pointing to the administration's unfreezing of Iranian funds to claim that Biden "betrayed Israel."

Blinken, however, reiterated Sunday that no money has been spent from the unfrozen funds intended for humanitarian purposes.

"So people are either misinformed or they're misinforming," he said. "And either way, it's wrong."

This story has been updated with additional information.

CNN's Kevin Liptak, MJ Lee and Jeremy Herb contributed to this report.