Citizens from Australia, Britain and Poland were among seven people working for celebrity chef Jose Andres' World Central Kitchen (WCK) who were killed in an Israeli airstrike in central Gaza on Monday, the NGO said.
The workers, who also included Palestinians and a dual citizen of the United States and Canada, were travelling in two armoured cars emblazoned with the WCK logo and another vehicle, WCK said in a statement.
Despite co-ordinating movements with the Israeli Defence Force, the convoy was hit as it was leaving its Deir al-Balah warehouse, after unloading more than 100 tons of humanitarian food aid brought to Gaza by sea, WCK said.
The Israeli military said it was doing a thorough review at the highest levels to understand the circumstances of what it called a tragic incident.
"The IDF makes extensive efforts to enable the safe delivery of humanitarian aid, and has been working closely with WCK in their vital efforts to provide food and humanitarian aid to the people of Gaza," the military said.
WCK was involved in the first shipment of aid to Gaza via a sea corridor from Cyprus in March. A second WCK maritime aid shipment of 332 tons arrived in Gaza early this week.
Prime Minister Benjamin Netanyahu said on Tuesday the killing of humanitarian aid workers' in Gaza in an IDF strike was an unintentional killing of innocents and would be investigated.