NEW YORK (JTA) — Human Rights Watch accused Israel of committing war crimes by attacking three U.N.-run schools in Gaza, killing more than 50 Palestinian civilians.

The New York-based organization issued a report Thursday documenting the attacks by the Israeli military during its 50-day Gaza operation against Hamas in July and August.

Attacks on the United Nations schools in Beit Hanoun and Jabalya killed 13 and 20 Palestinian civilians, respectively, and “did not appear to target a military objective or were otherwise indiscriminate,” according to the report. A third attack in Rafah that killed 12 Palestinian civilians was deemed “unlawfully disproportionate.”

The Israeli military claimed it was either targeting militants in the area of the schools or returning fire from Hamas fighters.

Israeli officials have yet to comment on the report, but in the past have rejected Human Rights Watch findings. Israel has said that the international human rights group should focus on Hamas putting Palestinian civilians in harm’s way by firing rockets from residential areas.

Israel has opened its own criminal investigations into the Gaza conflict, including one into the Beit Hanoun incident. However, Human Rights Watch claimed that “Israel has a long record of failing to undertake credible investigations into alleged war crimes.”

An open-ended cease-fire ended the latest Israel-Hamas conflict on Aug. 26.