The agency “protested strongly to the relevant authorities” to express “outrage and condemnation” over the presence of the structure.
“In accordance with UNRWA protocols, the agency immediately cordoned off the area and swiftly took the necessary measures to render the school safe, including permanently sealing the cavity,” it said.
“The presence of a man-made cavity underneath the grounds of an UNRWA school is a serious violation of the agency’s neutrality and a breach of international law. Moreover, it exposes children and agency staff to significant security and safety risks.”
Some observers and analysts of the Palestinian-Israeli conflict accuse Hamas of hiding weapons in civilian infrastructure.
Hamas has not commented on the UN findings, while a number of other militant groups operate in Gaza.
Last year, UNRWA said “a cavity and a possible tunnel” were found under the UNRWA Zaitoun Preparatory and Elementary Boys' School after an Israeli strike caused damage to the civilian facility.
Short of naming Hamas as the group responsible for the underground structures, UNRWA condemned “the existence and potential use by Palestinian armed groups of such tunnels”.
UNRWA provides support for 1.4 million Palestinian refugees in Gaza.