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.