Hamas said on Friday that it had agreed to free Edan Alexander, an American Israeli soldier who has been held in Gaza for 17 months, along with the remains of four other hostages with dual citizenship. Israel immediately cast doubt on the proposal’s viability, suggesting a deal was unlikely to be imminent.

In a statement, Hamas said it had formally declared its willingness to free Mr. Alexander and the others in a response to the latest cease-fire proposal offered by mediators, without specifying which ones. But the Palestinian armed group did not say what it was demanding in exchange for the captives or when it would turn the five over to Israel.

A senior Hamas official said the group’s offer stipulated that Israel release some Palestinian prisoners, restore aid to Gaza and enter talks on the cease-fire’s next phase in exchange for Mr. Alexander — believed to be the last surviving captive with American citizenship — and the remains of four other American-Israelis.

Hamas was willing to discuss the number and identities of the prisoners, the official said, as long as Israel accepted the principle of the offer for the five Americans. The official spoke on condition of anonymity to discuss sensitive diplomacy.

The office of Benjamin Netanyahu, the Israeli prime minister, argued Hamas was engaged in “manipulations” and “psychological warfare.” Israel had already accepted a separate U.S. offer that would see far more hostages released in exchange for an extension of the cease-fire, Mr. Netanyahu’s office said in a statement.

Hamas’s gesture appeared at least partially to be an appeal to President Trump, whose administration has repeatedly called for Mr. Alexander’s freedom. Last week, Adam Boehler — Trump’s pick for hostage envoy — met Hamas officials in Doha, Qatar, to negotiate the release of Mr. Alexander and the bodies of the four other Americans.

Hamas’s announcement could pose Mr. Netanyahu with a dilemma: rejecting an offer to bring home the living and dead Americans held in Gaza — even for a steep price — could antagonize members of the Trump administration who have clearly said they want Mr. Alexander and the bodies of the others to be brought home.

Mediators had hoped that Israel and Hamas would have agreed by now on the next steps in the Gaza cease-fire that began in January. The second phase is supposed to see a comprehensive end to Israel’s campaign against Hamas, a full Israeli withdrawal from Gaza and the release of the remaining living hostages for Palestinian prisoners.

But those talks have seen little progress given entrenched disputes between the two sides over the future of Gaza. Israel has demanded Hamas be removed from power, while Hamas has appeared loath to disband its battalions of armed fighters or send its leaders into exile. Mr. Netanyahu’s far-right allies have also agitated for a return to fighting against Hamas, meaning that ending the war could destabilize the prime minister’s grip on power.

The Trump administration’s direct meetings with Hamas sidestepped the moribund cease-fire efforts and broke with yearslong U.S. policy that aimed to isolate the group by refusing to talk to it directly. The talks prompted ire in the Israeli government, which Mr. Boehler acknowledged during a round of television interviews this week.

Marco Rubio, the secretary of state, later described Mr. Boehler’s meetings as a “one-off” that had yet to produce results.

But Hamas’s latest offer was similar to what Mr. Boehler proposed last week in Doha, which included a plan to jump-start negotiations for the second phase and bring home the remaining hostages, said a U.S. official, who spoke on condition of anonymity because they were not authorized to speak publicly. That proposal followed days of talks that Mr. Boehler conducted in close coordination with Steve Witkoff, Mr. Trump’s Middle East envoy, the official said.

Up to 24 living hostages and the bodies of more than 30 others are still in Gaza, according to the Israeli government. They include five Americans.

Hamas and other militant groups seized about 250 hostages during the Oct. 7, 2023, attack on southern Israel that ignited the war in Gaza. More than 100 have returned alive to Israel after deals with Hamas in exchange for Palestinian prisoners. Israeli soldiers retrieved the bodies of dozens of others during the ground invasion of Gaza.

In mid-January, Israel and Hamas signed off on the multiphase cease-fire, during which 30 hostages and the remains of eight others were exchanged for more than 1,000 Palestinian prisoners over the first six weeks of the truce. The release of others — like Mr. Alexander — was deferred until both sides concluded talks on the nebulous “second phase.”

Mr. Alexander grew up in Tenafly, N.J., to Israeli-born parents. After high school, he moved to Israel to enlist in the military; he was abducted from the post where he was stationed during the Hamas-led attack. Hamas published a hostage video featuring him last year.

The prospect that captives holding another nationality — like Mr. Alexander — might be prioritized has already prompted furious denunciation from the relatives of Israeli hostages held in Gaza. Many have called for a single agreement that frees all the remaining captives at once.

“If Israel insists on stopping in the middle and leaves its citizens behind — let every Israeli mother know that she must get her son a foreign passport, or else he’ll be abandoned,” the Hostage Families Forum, an advocacy group, said in a statement.