Senator Chuck Schumer said on Sunday that at least four Americans had been killed in Israeli attacks and U.S. officials expected the death toll to rise.
The Senate majority leader said in a statement that senior national security and State Department officials briefed him on the situation Sunday night.
"Sadly, we know Americans have been killed. The government tells us there have been four known incidents so far, but unfortunately we know the number of victims will grow," Schumer, a Democrat, said in a statement explain.
Earlier on Sunday, Secretary of State Antony Blinken said the United States was "working overtime" to verify reports of missing and dead Americans after Hamas launched an unprecedented attack on Israel.
"We have reports that several Americans have been killed. We are working overtime to check. At the same time, there are reports that Americans are missing and we are working to verify those reports," Blinken told CNN on "State of the Union." Dana Bash, CNN.
Earlier on Sunday, CNN reported that at least three Americans had been killed, according to an internal U.S. government memo. The memo said the State Department was aware of other Americans who were unaccounted for.
CNN has reached out to the State Department for comment.
Israeli Strategic Affairs Minister Ron Demer told Bash on Sunday that the Americans were among "dozens of hostages" being held in Gaza.
Meanwhile, the U.S. Embassy in Jerusalem has updated its contingency plans "for any possible land or air evacuation," the memo said. On Saturday, the embassy issued an order to provide accommodation for its staff.
A massive surprise attack by Gaza militants on Saturday killed hundreds of Israelis, prompting Prime Minister Benjamin Netanyahu to vow retaliation and launch deadly Israeli retaliatory airstrikes.
It is unclear what the full Israeli response will be. An Israeli military spokesman hinted that Israel may try to fully control Gaza for the first time since 2005 and that more than 20 communities near Gaza will be evacuated.
The top U.S. diplomat added that President Joe Biden's directive is to "make sure that at this moment we provide Israel with everything it needs to respond to Hamas attacks."
Biden said at the White House on Saturday that his administration's support for Israel's security is "rock solid and unwavering."
In a conversation with Netanyahu on Sunday, Biden said help was "on the way" and more help was coming.
The United States will send an aircraft carrier group to the eastern Mediterranean as a "deterrence post," Defense Secretary Lloyd Austin announced on Sunday. Austin also said in the statement that the United States would "quickly provide additional equipment and resources to the Israel Defense Forces."
Still, the president and his top aides are grappling with a complex diplomatic situation unlike any previous conflict between Israel and the Palestinians.
Republican presidential candidates criticized the Biden administration. Former President Donald Trump, the front-runner for the Republican nomination, pointed to the administration's release of Iranian funds and claimed Biden "betrayed Israel."
However, Blinken reiterated on Sunday that none of the unfrozen funds intended for humanitarian purposes had been spent.
"Either people are being misled or they are providing wrong information themselves," he said. "Either way it's wrong."
This story has been updated with additional information.