Hersh Goldberg-Polin was at the Nova music festival in a forested area close to the Gaza Strip when Hamas-led gunmen stormed Israel's border fence, killing about 1,200 people and seizing 253 hostages, according to Israeli tallies.

More than 360 of those killed were attending the festival event.

Hersh managed to get several kilometres away from the festival site in a car, before seeking refuge in a roadside bomb shelter with several dozen others. But outside, Hamas gunmen gathered and began throwing in grenades.

Before Wednesday, the last time he had been seen alive was in a Hamas video that showed him being loaded on to a pick-up truck, with part of his left arm missing.

In the new video, posted to Hamas's Telegram account, Hersh lifts his arm to the camera to show a healed stump where his hand should be. He criticises Israel's military campaign in Gaza before speaking directly to his parents and two sisters, urging them to keep campaigning for his release.

"We're clearly very concerned about the injury," Mrs Goldberg-Polin told me.

"We've had several surgeons who have seen the video that say he needs another surgery immediately. If he was treated, it was an emergency treatment. He needs a second surgery at least."

"He is considered in a fragile and compromised medical state, and so we're concerned about that."