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

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

On Monday, the Palestinian territory's Civil Defence agency said health workers had uncovered more than 200 bodies of people killed and buried at Nasser Hospital, which was besieged by Israeli troops last month.

In early April the World Health Organization said Al-Shifa had been destroyed by an Israeli siege, leaving an "empty shell" containing many bodies.

The UN rights office on Tuesday 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 Gazan medical facilities as command centers and holding hostages abducted during its attack inside Israel on October 7.

Hamas has denied those claims.

  'War crime' 

 "Hospitals are entitled to very special protection under international humanitarian law," Turk pointed out.

"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 had transpired at the two hospitals.

But spokeswoman Ravina Shamdasani said efforts were underway to corroborate reports and details given by Gaza authorities.

The latter says 283 bodies were recovered from Nasser Hospital, including 42 that had been identified.

"Victims had reportedly been buried deep in the ground and covered with waste," she told reporters in Geneva.

Older people, women, and wounded were reportedly among the dead, she said.

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

As for Al-Shifa, the Israeli army has said around 200 Palestinians were killed during its military operation at the hospital

Shamdasani pointed to reports indicating that this toll "may be an underestimate".

Around 30 bodies were reported buried in two graves in the courtyard of Al-Shifa Hospital.

"And there are reports that the hands of some of these bodies were also tied," Shamdasani said.

So far, she said, the UN "can't corroborate the exact figures" of people killed at the two hospitals, underlining: "This is why we are stressing the need for international investigations".

"There have been multiple bodies discovered," she said.

The reports that some had their hands tied indicated "serious violations" of international law, she added.

"These need to be subjected to further investigation... They can't just be more reports in this horrific war that just pass under the radar."