The United Nations called on Tuesday for an international investigation into reports of mass graves at two Gaza hospitals destroyed in Israeli sieges, saying war crimes might have been committed.

The UN rights office said it was "horrified" by the destruction of Gaza's biggest hospital, Al-Shifa in Gaza City, and its second largest, the Nasser Medical Complex in Khan Younis.

Gaza's Civil Defence agency said on Tuesday that health workers had uncovered nearly 340 bodies, updating an earlier figure, of Palestinians allegedly killed and buried by Israeli forces at the Nasser complex.

Israel's army dismissed claims that its troops had buried bodies during its operation at Nasser as "baseless and unfounded."

The UN rights office demanded "independent, effective and transparent investigations into the deaths."

"Given the prevailing climate of impunity, this should include international investigators," UN rights chief Volker Turk said in a statement.

Hospitals, which are protected under international law, have repeatedly come under Israeli bombardment over more than six months of war in Gaza.

Israel has accused Palestinian militant group Hamas of using medical facilities as command centres and to hold hostages abducted during its attack inside Israel on October 7.

Hamas has denied the claims.

"Hospitals are entitled to very special protection under international humanitarian law," Turk said. "And the intentional killing of civilians, detainees and others who are hors de combat is a war crime."

The UN rights office said it did not have access to independent information as to what happened at the two hospitals.

But spokeswoman Ravina Shamdasani said efforts were under way to corroborate reports and details given by Gazan authorities.

"Victims had reportedly been buried deep in the ground and covered with waste," she told reporters in Geneva, adding that older people, women and wounded were reportedly among the dead.

Others were allegedly "found with their hands tied and stripped of their clothes." (AFP)