Hamas said “significant efforts and special equipment” are needed to recover the bodies of the remaining Israeli hostages in Gaza.
The terror group said it had returned all the hostages that it could.
As part of the first phase of the Gaza peace plan, Hamas had to return 20 living hostages and the bodies of another 28 hostages within 72 hours.
The living hostages have been returned, as well as nine bodies - but Hamas said it needs specialist equipment to find the others.
It could take weeks or months to locate them.
The admission has sparked fury in Israel where there is anger over what Israeli Defense Minister Israel Katz called “a failure to uphold commitments."
Israeli sources have said the country's leadership is frustrated over the slow pace of the return and is considering several options if the delays continue.
It comes on top of fury a body that was not a hostage was handed over to Israel.
Onlookers are concerned the failure to return the bodies risks the fragile Gaza ceasefire agreement between Hamas and Israel.
But the US has downplayed the threat to the ceasefire by the delays.
Advisers told the BBC there was "understanding" to get all the live hostages, which Hamas did "honour".
We have a "mechanism in place", and are working with mediators to get as many bodies "as possible" out, they told the broadcaster.
“We continue to hear from them that they intend to honour the deal," an adviser said.
Israel had earlier warned it could keep the Rafah border crossing shut and reduce aid into the Palestinian enclave as Hamas was returning the bodies of dead hostages too slowly.
Israel’s military aid agency COGAT said on Thursday preparations are ongoing with Egypt to open the Rafah crossing with Gaza for the movement of people, with the date to be announced at a later stage.
COGAT, the arm of the Israeli military that oversees aid flows into the Gaza Strip, said humanitarian aid continued to enter the territory via the Kerem Shalom crossing with Israel, and at other crossings.
"It should be emphasised that humanitarian aid will not pass through the Rafah crossing. This was never agreed upon at any stage," COGAT added in a statement sent to Reuters.
