Some 20 rockets were found Wednesday in a school in Gaza operated by the United Nations Relief and Works Agency, the organization confirmed Thursday.

The weapons were found in “the course of the regular inspection of its premises,” UNRWA said in a statement, adding that the school was vacant.

“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,” the statement read.

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The discovery would seem to confirm Israel’s oft-repeated claim that Hamas and other Gazan terror groups use civilian infrastructure to hide weapons.

“Yet again, Gaza terrorists abuse UN 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 spokesperson Yigal Palmor told The Times of Israel. “We expect the UN 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.”

This incident was “the first of its kind in Gaza,” the organization said, adding that it “endangered civilians including staff and put at risk UNRWA’s vital mission to assist and protect Palestinian refugees in Gaza.”

UNRWA informed all officials immediately after discovering the rockets, and “successfully took all necessary measures for the removal of the objects in order to preserve the safety and security of the school.”

Officials from the agency, which has a reputation in Jerusalem of being exceedingly critical of Israeli policies vis-à-vis the Palestinians, have been summoned to the Foreign Ministry. They have apologized to Israel for the incident, Foreign Minister Avigdor Liberman said.

UNRWA has launched a “comprehensive investigation” into the incidence, the statement said.

The agency, charged with taking care of the welfare of Palestinian refugees, 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,” its statement read. “UNRWA will uphold and further reinforce its procedures.”