Israel on Monday accused Hamas of violating the first phase of the Donald Trump-backed ceasefire deal, claiming that the group released bodies of only four of the 28 slain hostages.
The Hostage and Missing Families Forum sought to halt the implementation of the ceasefire deal until the "full and complete release of all the fallen".
"The violation of the agreement by Hamas must be met with a very severe response from the government and the mediators," the forum said. "An agreement must be respected by both sides, if Hamas does not fulfill its part, Israel should not fulfill its part either."
The bodies of Yossi Sharabi, Daniel Perez, Guy Illouz and Bipin Joshi were handed over to Israel. Hamas did not clarify when the bodies of the remaining 24 dead hostages would be released.
Israel's allegations come after Hamas returned the final 20 living hostages.
Yossi Sharabi, 53, was the brother of Eli Sharabi who was freed by Hamas earlier. They were kidnapped from Kibbutz Be'eri on October 7, 2023.
Daniel Perez, 22, was a platoon commander in the 7th Armoured Brigade's 77th Battalion.
Guy Illouz was abducted from the Nova music festival and died at a Gaza hospital after allegedly not receiving medical treatment.
Bipin Joshi was a Nepali agricultural student but his death was not previously confirmed by Israel.
After the caskets of the dead hostages are examined by sappers "for security purposes", the civilian hostages would be taken to the Abu Kabir forensic institute for identification. The bodies of slain soldiers will be shifted to IDF's Shura Camp.
Hamas had told mediators that they were not aware of the whereabouts of the bodies of 28 dead hostages.