UNITED NATIONS (AP) — The United Nations called Tuesday for “a clear, transparent and credible investigation” of mass graves uncovered at two major hospitals in war-torn Gaza that were raided by Israeli troops.

Credible investigators must have access to the sites, U.N. spokesman Stephane Dujarric told reporters, and added that more journalists need to be able to work safely in Gaza to report on the facts.

Earlier Tuesday, U.N. human rights chief Volker Türk said he was “horrified” by the destruction of the Shifa medical center in Gaza City and Nasser Hospital in the southern city of Khan Younis as well as the reported discovery of mass graves in and around the facilities after the Israelis left.

He called for independent and transparent investigations into the deaths, saying that “given the prevailing climate of impunity, this should include international investigators.”