The family of Evyatar David, an Israeli held hostage in Gaza, saw him for the first time in months on Friday evening in a video circulated by his Hamas captors. He was emaciated and sallow in what appeared to be an underground tunnel.

The video prompted an outpouring of fear and horror among the families of the dozens of Israeli hostages still held in Gaza nearly two years after the Oct. 7, 2023, attack on Israel that killed about 1,200 people and ignited the war.

“Our brothers are turning into skin and bones at this very moment,” Einav Zangauker, whose son Matan was one of about 250 people taken hostage during the Hamas assault, said at a rally on Saturday. His son is believed to still be alive.

More than 100 hostages were freed during two short-lived cease-fires, the last of which ended in March. Israeli forces have found the corpses of some other hostages as they swept through Gaza. Israel believes that about 20 hostages are still alive and nearly 30 are presumed dead.

Cease-fire talks between Israel and Hamas to free more hostages in exchange for Palestinian prisoners have stalled. Last month, Israeli and American negotiators left Doha, Qatar, where most of the talks have been held, without a breakthrough.

On Saturday, President Trump’s Middle East envoy, Steve Witkoff, met with the families of Israeli hostages in Tel Aviv. He reiterated Mr. Trump’s commitment to bringing home the remaining captives and said, “those who are alive must be kept alive,” according to the Hostage Families Forum, an advocacy group.