Hamas's armed wing said on Saturday the fate of Israeli-American soldier Edan Alexander was unknown after the group found the guard who was holding him dead.
President Donald Trump's Middle East envoy Steve Witkoff told reporters at the White House in March that gaining the release of Alexander, believed to be the last living American prisoner held by Hamas in Gaza, was a "top priority".
Alexander, 21, is a New Jersey native and a soldier in the Israeli army.
On Tuesday, Hamas said it had lost contact with the group holding him after the Israeli army attacked their location.
"The fate of the prisoner and the rest of the captors remains unknown," said Al-Qassam Brigades' spokesperson Abu Obeida, blaming Israel for the killing of the guard and the fate of Alexander.
"We are trying to protect all the captives and preserve their lives ... but their lives are in danger because of the criminal bombings by the enemy's army," he said.
There was no immediate comment by the Israeli army on Hamas' statement.
The release of Alexander was at the centre of talks held between Hamas leaders and US hostage envoy Adam Boehler last month.
The Palestinian group released 38 captives under a brief ceasefire that began on 19 January.
Netanyahu statement
Israel resumed its brutal offensive in Gaza in March after refusing to participate in talks to enter the second phase of the ceasefire agreed in January.
Israeli attacks since then have killed almost 1,800 people, according to the Gaza health ministry.
Hamas has said it will only free captives as part of a deal to end the war and has rejected Israel's demands to lay down its arms.
Late on Thursday, senior Hamas official Khalil Al-Hayya said the movement was willing to swap all remaining 59 captives for Palestinians jailed in Israel in return for an end to the war and the reconstruction of Gaza.
But he dismissed an Israeli demand for the group to surrender its weapons, which he said was imposing "impossible conditions".
Israel has not responded formally to Al-Hayya's comments, but ministers have said repeatedly that Hamas must be disarmed completely and can play no role in the future governance of Gaza. Israeli Prime Minister Benjamin Netanyahu is scheduled to give a statement later on Saturday.
Hamas on Saturday also released an undated and edited video of Israeli prisoner Elkana Bohbot. Hamas has released several videos over the course of the war of captives begging to be released. Israeli officials have dismissed past videos as propaganda.
The Israeli military has killed more than 51,000 Palestinians and completely devastated the territory in the 18-month war.
It has imposed a total blockade on all goods entering Gaza for more than six weeks, imposing starvation conditions on its 2.2 million residents.
(Reuters and The New Arab Staff)