“Be prepared. Soon, your children will return in black coffins with their bodies torn apart by shrapnel from your army’s missiles,” the message warns.
Hamas’s armed wing released a video on Saturday showing Alexander alive, in which he criticised the Israeli government for failing to secure his release.
Alexander appeared to be speaking under duress in the footage, making frequent hand gestures as he criticised Netanyahu’s government.
AFP was unable to determine when the video was filmed.
Alexander was serving in an elite infantry unit on the Gaza border when he was abducted by Palestinian militants during their October 2023 attack.
The soldier, who turned 21 in captivity, was born in Tel Aviv and grew up in the US state of New Jersey, returning to Israel after high school to join the army.
