The United Nations humanitarian agency that serves those claiming Palestinian refugee status said Thursday that it had discovered 20 rockets hidden by terrorists in one of its schools in Gaza.

The discovery would appear to bolster repeated assertions by the Israeli military that Hamas and other terrorist groups in Gaza are hiding their weapons stockpiles and firing them among civilians, thus turning the local population into “human shields.”

Smoke from rockets fired from a northern neighborhood of Gaza City are seen after being launched toward Israel on July 17, 2014. (Photo: Thomas Coex /AFP/Getty Images)

Smoke from rockets fired from a northern neighborhood of Gaza City toward Israel is seen, July 17, 2014. (Thomas Coex/AFP/Getty Images)

In a statement, the United Nations Relief and Works Agency insisted this was the first time weapons had been found in one of its installations:

Yesterday, in the course of the regular inspection of its premises, UNRWA discovered approximately 20 rockets hidden in a vacant school in the Gaza Strip. UNRWA strongly condemns the group or groups responsible for placing the weapons in one of its installations. This is a flagrant violation of the inviolability of its premises under international law. This incident, which is the first of its kind in Gaza, endangered civilians including staff and put at risk UNRWA’s vital mission to assist and protect Palestine refugees in Gaza.

The U.N. agency would not say who placed the rockets there, or to whom it complained other than saying it had informed “the relevant parties” and “took all necessary measures” to remove the rockets.

“UNRWA has strong, established procedures to maintain the neutrality of all its premises, including a strict no-weapons policy and routine inspections of its installations, to ensure they are only used for humanitarian purposes,” it said.

Though the agency asserted this was the first such occurrence, the Israeli Foreign Ministry insisted hiding the weapons in this manner has been a repeated problem.

“Yet again, Gaza terrorists abuse U.N. facilities to carry out their violent activities. Hamas and other terror groups are determined to put civilians in harm’s way and will respect nothing in their violent frenzy,” Foreign Ministry spokesman Yigal Palmor told the Times of Israel. “We expect the U.N. and the international community to condemn and to act strongly against this brazen violation of international humanitarian law, which endangers children and UN humanitarian activities.”

The U.N. agency has a reputation of being intensely critical of Israeli policy. Earlier this week, spokesman Chris Gunness was criticized by Israel supporters for suggesting reporters in Gaza interview a Norwegian doctor who once famously in his country justified the 9/11 attacks on the U.S.

An estimated 22,000 Palestinians have fled their homes in the past week at the instruction of the IDF, and moved to UNRWA facilities to seek safety. With the IDF focused on bombing Hamas facilities and storage depots, terrorist groups know hiding their weapons at a U.N. compound would offer them a measure of security.

Over the weekend, the military wing associated with Palestinian Authority President Mahmoud Abbas’ Fatah party circulated video showing one of its militants firing rockets on a launcher that was camouflaged by an object resembling a kind of food cart seen widely in Palestinian markets. The terrorist fired his rockets within yards of a house.