Video obtained by Reuters showed a large hole in the roof of a four-wheel-drive vehicle belonging to the World Central Kitchen, a charity founded by celebrity chef Jose Andres.

Paramedics were seen moving bodies into a hospital and displaying the passports of three of those killed.

U.S. National Security Council Coordinator for Strategic Communications John Kirby said the White House was outraged by the attack.

"I think by saying 'we're 'outraged,' I think you can fairly characterize that as condemning the strike itself, of course. Nobody wants to see this kind of violence happen to humanitarian aid workers who - as was noted earlier - were doing all the right things."

The airstrike killed citizens of Australia, Britain and Poland as well as Palestinians and a dual citizen of the United States and Canada.

Jessica Olney was a friend of Australian aid worker Zomi Frankcom, who was killed in the airstrike.

"There's no words. You know, these are some of the best people out there doing the most important work. And people like Zomi are so selfless and only doing what they do because of their love for humanity. There's no other reason... It makes me feel like humanity has reached a new low to see something like this happen."

Israel's military expressed "sincere sorrow" over the incident. Here's Prime Minister Benjamin Netanyahu.

"Unfortunately, in the last day there was a tragic case of our forces unintentionally hitting innocent people in the Gaza Strip. 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."

The incident ratcheted up international pressure for steps to ease the disastrous humanitarian situation in Gaza nearly six months into Israel's siege and invasion of the Palestinian enclave, including from UN Secretary General Antonio Guterres.

"This is unconscionable, but it is an inevitable result of the way the war is being conducted." // "It demonstrates yet again the urgent needs for an immediate humanitarian ceasefire, the unconditional release of all hostages, and the expansion of humanitarian aid into Gaza, as the Security Council demanded in its resolution last week. The resolution must be implemented without delay."

The Israeli military pledged an investigation into the deaths of the aid workers by "an independent, professional and expert body."