UN human rights chief Volker Turk called Tuesday for an independent investigation into reports that Palestinian armed groups summarily executed six Israeli hostages.
Israel's military said the six were all captured alive during Hamas's October 7 attack on southern Israel that triggered the war, and were shot dead shortly before troops found them.
The military announced Sunday it had recovered the bodies from a tunnel in the Gaza Strip.
Israeli Prime Minister Benjamin Netanyahu said they were "executed" with a bullet "to the head".
"We are horrified by reports that Palestinian armed groups summarily executed six Israeli hostages, which would constitute a war crime," the UN Human Rights Office said on X.
It added that Turk, the United Nations high commissioner for human rights, "calls for independent, impartial and transparent investigation and for perpetrators to be held to account".
The October 7 attack on Israel resulted in the deaths of 1,205 people, mostly civilians and including hostages killed in captivity, according to an AFP tally based on Israeli official figures.
Gripped by grief and fury after six dead hostages were recovered from Gaza, Israelis took to the streets on Sunday and Monday to ramp up pressure on their government to secure the release of the remaining captives.
Of 251 hostages seized by Palestinian militants during the attack, 97 remain in Gaza including 33 the Israeli military says are dead. Scores were released during a one-week truce in November -- the only one so far.
Hamas's armed wing said on Monday that hostages would return to Israel "inside coffins" if military pressure continued.
Israel's campaign against Hamas has so far killed at least 40,819 people in Gaza, according to the Hamas-run territory's health ministry. The UN rights office says most of the dead are women and children.