Army engineers took reporters for foreign news outlets through the passages at a time of crisis for UNRWA, which has launched an internal probe and seen a string of donor countries freeze funding over allegations last month by Israel that some of its staff doubled as Hamas operatives.

The Palestinians have accused Israel of falsifying information to tarnish UNRWA, which employs 13,000 people in the Gaza Strip and has been a lifeline for the aid-dependent population for years. The agency runs schools, primary healthcare clinics and other social services, and distributes aid, describing its activities as purely humanitarian.

UNRWA Headquarters is in Gaza City, among northern areas that Israeli troops and tanks overran early in the four-month-old war against the governing Islamist faction Hamas, sending hundreds of thousands of civilians fleeing southward.

Reporters on the closely escorted trip entered a shaft next to a school on the periphery of the U.N. compound, descending to the concrete-lined tunnel. Twenty minutes of walking through the stifling hot, narrow and occasionally winding passage brought them underneath UNRWA Headquarters, an army lieutenant-colonel leading the tour said.

The tunnel, which the military said was 700 metres long and 18 metres deep, bifurcated at times, revealing side-rooms. There was an office space, with steel safes that had been opened and emptied. There was a tiled toilet. One large chamber was packed with computer servers, another with industrial battery stacks.

"Everything is conducted from here. All the energy for the tunnels, which you walked through them are powered from here," said the lieutenant-colonel, who gave only his first name, Ido.

"This is one of the central commands of the intelligence. This place is one of the Hamas intelligence units, where they commanded most of the combat."

But Ido said Hamas appeared to have evacuated in the face of the Israeli advance, preemptively cutting off communications cables that, in an above-ground part of the tour, he showed running through the floor of the UNRWA Headquarters' basement.

It appeared that heavy Israeli barrages and sustained winter rains may also have played a part in the departure: Several stretches of the tunnel were clogged with dislodged sand and knee-high water.

In a statement, UNRWA said it had vacated the headquarters on Oct. 12, five days after the war began, and was therefore "unable to confirm or otherwise comment" on the Israeli finding.

"UNRWA ... does not have the military and security expertise nor the capacity to undertake military inspections of what is or might be under its premises," the statement said.

"In the past, whenever (a) suspicious cavity was found close to or under UNRWA premises, protest letters were promptly filed to parties to the conflict, including both the de facto authorities in Gaza (Hamas) and the Israeli authorities."

UNRWA's supporters say it is the only agency with the means of aiding Palestinians in deepening humanitarian distress. Israel says the agency is "perforated by Hamas" and must be replaced. Hamas has denied operating in civilian facilities.

"We know that they (Hamas) have people working in UNRWA. We want every international organization to work in Gaza. That is not a problem. Our problem is the Hamas," Ido told reporters.

Lack of cellphone reception in the tunnel made geolocating it as under UNRWA Headquarters impossible. Instead, reporters were asked to put personal items in a bucket that was lowered by rope into a vertical hole on the grounds of the headquarters. They were reunited with the still-tethered items during the tunnel tour.

As a condition of taking journalists on the trip, the Israeli military did not allow photographs of military intelligence such as maps or certain equipment in the convoy of armoured vehicles they traveled in. It also requested approval before transmission of photographs and video footage taken on the trip.

(Writing by Dan Williams; Editing by Frances Kerry)

By Dylan Martinez