Israel announced Wednesday that one of the four bodies delivered by Hamas overnight was not that of a recognized hostage, intensifying tensions.

Hamas transferred four coffins to Israel late Tuesday, each believed to contain the remains of Israeli captives held in Gaza. However, on Wednesday, the Israel Defense Forces (IDF) disclosed that forensic examinations at the National Institute of Forensic Medicine determined that “the fourth body supplied … does not correspond to any of the hostages.” Israel affirmed that “Hamas is obligated to make all necessary efforts to return the deceased hostages.”

The three other bodies were identified via DNA testing as Tamir Nimrodi, Uriel Baruch, and Eitan Levi. Nimrodi, an 18-year-old soldier at the time of his passing, was reportedly killed by Israeli airstrikes while in Gaza, according to statements from the Hostages and Missing Families Forum. The forum asserted that “Tamir was abducted alive from his base and killed by IDF bombings during his captivity.” Baruch, aged 35, and Levi, aged 53, perished during the October 7, 2023, Hamas terror attack on Israel and were subsequently transported into Gaza.

Following the revelation of the body’s misidentification, Israel cautioned it would offer no concessions in its efforts to secure the repatriation of the remaining captives. A government spokesperson stated, “We will not compromise on this, and we will spare no effort until all our fallen hostages are returned, every last one of them,” reiterating Hamas’s responsibility to fulfill its commitments to the mediating parties.

Under the ceasefire agreement, Israel is to return 15 Palestinian bodies for every Israeli body handed over by Hamas. To date, Israel has transferred 90 bodies to Gaza. The International Committee of the Red Cross (ICRC) is facilitating these civilian exchanges. The head of the ICRC delegation in Israel and the Occupied Territories said, “We know that the families won’t give up … and neither will we — we are prepared to fulfill our role for as long as it takes.”

As of now, Hamas has repatriated only seven of the 28 bodies identified as deceased hostages. Earlier on Tuesday, the bodies of IDF Captain Daniel Peretz, Yossi Sharabi, Guy Iloz, and Nepali citizen Bipin Joshi were returned. However, Israel’s military had previously warned Hamas that it would reduce aid flow if bodies were not handed over by a Monday deadline. Hamas has stated it has faced difficulties locating the bodies of some hostages, believed to be buried under rubble.

The United Nations expressed alarm over Israel’s decision to withhold a portion of the humanitarian aid promised under the agreement. Israeli officials informed the UN Tuesday that they would impose aid sanctions, halving the volume of trucks permitted into Gaza. COGAT, the Israeli body responsible for coordination in Gaza and the West Bank, confirmed these measures.

U.N. Secretary-General Antonio Guterres and U.N. humanitarian chief Tom Fletcher urged both parties to adhere to their commitments. Fletcher emphasized that withholding aid from civilians should never be used as a bargaining tool, asserting that ensuring its entry is a legal obligation. TIME has reached out to COGAT for comment regarding the closures.

In Gaza, medical organizations condemned Israel’s withholding of assistance. Fikr Shalltoot, director of Medical Aid for Palestinians in Gaza, stated in a statement shared with TIME, “It is frankly outrageous that life-saving humanitarian aid … and the lives of Palestinians are used by Israel as a bargaining chip.”

On Monday, individuals held in Gaza were transferred into Israeli custody. In exchange, nearly detainees and prisoners who had been apprehended from Gaza since the war began were released into Gaza and the West Bank. Witnesses described many released hostages as dangerously thin; many Palestinian returnees exhibited wounds and mobility injuries.

On the ground in Gaza, the fragile calm is already strained by fears of internal unrest. Rival armed factions are fighting for influence in the post-war vacuum. One casualty was Palestinian journalist Saleh Aljafarawi, 28, who had covered the war; he was reportedly shot by a militia and buried the same day his brother Naji was released from an Israeli prison under the exchange deal.

As the ceasefire precariously endures, the misidentification of the fourth body has raised questions about Hamas’s ability—and intent—to meet its commitments under the truce brokered by U.S. President Donald Trump. Israeli officials say the group must now provide a full accounting for all deceased hostages, while mediators work to preserve a deal that has so far halted two years of devastating conflict.