US President Joe Biden said on Tuesday he was “outraged and heartbroken” by an Israeli airstrike in Gaza that killed seven people working for World Central Kitchen (WCK) and he called on Israel to do more to protect aid workers, Reuters reports.

Israel’s investigation of the incident “must be swift, it must bring accountability, and its findings must be made public,” Biden said in a statement.

“Even more tragically, this is not a stand-alone incident,” he said. “This conflict has been one of the worst in recent memory in terms of how many aid workers have been killed.”

The strike on the WCK convoy killed citizens of Australia, Britain and Poland as well as Palestinians and a dual citizen of the U.S. and Canada. Israeli Prime Minister Benjamin Netanyahu said earlier that the strike was tragic and unintended, and the Israeli military pledged an independent inquiry.

Biden said Israel has not done enough to protect aid workers or civilians in Gaza.

“The United States has repeatedly urged Israel to deconflict their military operations against Hamas with humanitarian operations, in order to avoid civilian casualties,” he said.