UNRWA said in response that it had not operated from the base since October 12 — five days after Hamas fighters attacked southern Israel — and called for an independent investigation.

Hamas has previously denied Israeli claims that it has dug an extensive network of tunnels under schools, hospitals and other civilian infrastructure as cover for its activities.

UNRWA is already under scrutiny after it sacked a number of staff last month following Israeli allegations that they took part in the deadly October 7 attacks by Hamas.

“The shaft led to an underground terror tunnel that served as a significant asset of Hamas’s military intelligence and passed under the building that serves as UNRWA’s main headquarters in the Gaza Strip,” they added in a statement.

“Electrical infrastructure” in the tunnel — 700 metres (765 yards) long and 18 metres underground — “connected” to the agency’s HQ, “indicating that UNRWA’s facilities supplied the tunnel with electricity”, they said.

Documents and a stash of weapons in the UN compound itself “confirmed that the offices had in fact also been used by Hamas terrorists”, the joint statement said.

“We have not used that compound since we left it nor are we aware of any activity that may have taken place there,” it added.

The compound was last inspected in September 2023, a statement said.

Any suspicious activity found near or under any UNRWA premises was previously reported to the authorities in Hamas-controlled Gaza and the Israelis, and also made public, it added.

“The Israeli authorities have not informed UNRWA officially about the alleged tunnel.”

The UN has launched two separate probes into UNRWA, the first into the Israeli claims of staff participation on October 7, and the other a review of its overall political neutrality.