A United Nations group providing humanitarian relief in the Gaza Strip said they found rockets hidden by militants inside a vacant school Tuesday, adding to complaints that Hamas is hiding and launching weapons in the vicinity of civilian structures.

The U.N. Relief and Works Agency (UNRWA) said the rockets were found in between two other UNRWA schools that are being used to host 1,500 displaced people.

“The agency immediately informed the relevant parties and is pursuing all possible measures for the removal of the objects in order to preserve the safety and security of the school,” the agency said in a statement. “UNRWA will launch a comprehensive investigation into the circumstances surrounding this incident.”

As soon as the rockets were discovered, UNRWA staff withdrew from the area.

The discovery came after Israel Defense Forces (IDF) accused Hamas of using the Gaza City neighborhood of Shijaiyeh as “fortress for its weapons,” according to The Algemeiner.

The IDF released aerial photos Sunday of rocket launch sites inside Shijaiyeh. The pictures claimed Hamas launched rockets from inside and around a mosque, a hospital, a children’s playground and a cemetery.

The IDF also said the neighborhood is “saturated with Hamas observation points and is home to a vast network of tunnels used for attacking Israel.”

On Monday, the IDF released a video of rocket launchers found next to an agricultural school in Beit Hanoun. Some of the rockets were still in the launchers, ready to be fired.