Citizens from Australia, Britain and Poland were among seven people working for celebrity chef Jose Andres' World Central Kitchen 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.
"This is not only an attack against WCK, this is an attack on humanitarian organisations showing up in the most dire of situations where food is being used as a weapon of war," said Erin Gore, chief executive of World Central Kitchen.
"This is unforgivable."
Prime Minister Benjamin Netanyahu, on Tuesday, described the incident as tragic and unintended.
"This happens in wartime. We are thoroughly looking into it, are in contact with the governments (of the foreigners among the dead) and will do everything to ensure it does not happen again," Netanyahu said in a video statement.
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.
Andres, who started WCK in 2010 by sending cooks and food to Haiti after an earthquake, earlier said he was heartbroken and grieving for the families and friends of those who died.
"The Israeli government needs to stop this indiscriminate killing," he said on social media.
"It needs to stop restricting humanitarian aid, stop killing civilians and aid workers, and stop using food as a weapon. No more innocent lives lost. Peace starts with our shared humanity. It needs to start now."
In a statement, Hamas said the attack aimed to terrorise workers of international humanitarian agencies, deterring them from their missions.
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