"We are waiting for a much more detailed clarification of what the causes have been, bearing in mind that the Israeli government knew about the actions and the itinerary of this NGO on the ground in Gaza," Sanchez told a Doha news conference at the end of a three-nation tour of the Middle East.

"It seems to me absolutely unacceptable, insufficient," he added when asked about Netanyahu's statements about the tragedy.

US-based World Central Kitchen -- founded by Spanish-American celebrity chef Jose Andres -- said a "targeted attack" by Israeli forces on Monday had killed the seven aid workers, a group that included Australian, British, Palestinian, Polish and US-Canadian employees.

Netanyahu later admitted Israel's military had "unintentionally" killed them in an air strike.

He said it was a "tragic case" that would be investigated "right to the end" but stopped short of apologising for the deaths.

The Israeli army said Tuesday it would hold an investigation into the air strike and pledged to "share our findings transparently".