Four Americans have been killed in the attacks in Israel close to the Gaza border and the death toll is expected to rise, sources familiar with a Sunday briefing to key House committees told CNN.

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.”