More than 20 people onboard a flotilla carrying aid for the Palestinian territory of Gaza were killed and 60 have been injured when security forces boarded the boats Monday.
Soldiers boarded six ships, the military said, after the flotilla refused to heed warnings to dock at Israel's Ashdod port, where supplies would be unloaded and transferred to Gaza.
"At about 4:30 am, Israeli commandos dropped from helicopter onto deck of Turkish ship, immediately opened fire on unarmed civilians," a post on the group's Twitter page said.
Video aired on CNN sister network CNN Turk showed what appeared to be soldiers rappelling onto the deck of a ship.
Turkish media reported two dead and 50 injured in the alleged incident as a flotilla of six ships approached the Gaza coast. Free Gaza co-founder Greta Berlin told Al Jazeera English that its lawyers in Israel informed her that 20 people have been killed and many others injured. CNN could not immediately confirmed the reports.
In Istanbul, Turkey, the Foreign Ministry has summoned the Israeli ambassador for an explanation
