The Israeli military on Sunday said it had uncovered a tunnel under Gaza's Al-Shifa hospital that stretched 55-metres beneath the war-torn complex where troops have been conducting a major operation.

"IDF troops exposed a 55-meter-long terror tunnel 10 metres deep underneath the Shifa hospital complex," which ran under the hospital and ended at a blast door, an army statement said.

Al-Shifa hospital has become a focal point for Israeli operations, with the army claiming Hamas uses it as a base. Gaza's Hamas rulers and medical staff at the hospital have denied the accusations.

Military spokesman Daniel Hagari told reporters the entrance was uncovered when a military bulldozer knocked down the outside wall of the hospital complex and found a fortified shaft with a spiral staircase descending 10 metres (yards).

"It's a huge one which has metal (spiral) stairs, then it goes along for 55 metres.. and reaches a blast door," said Hagari, indicating troops had not yet tried to open the door for fear it would be booby trapped.

Beyond the door, intelligence suggested either the tunnel would either split or there would be "a big room for command and control", he added, saying troops would continue searching the area as there could be access shafts from nearby houses.

Troops also discovered a white Toyota pickup truck containing "RPGs, explosives, grenades, AK47s," inside the hospital compound which Hagari said was "identical" to those used by Hamas militants when they attacked Israel on October 7.

When the army first entered Al-Shifa on Wednesday, there were "around 2,000 people inside", Hagari said, describing the huge hospital complex as covering an area of "more than 20 acres", or eight hectares.

Israel has pounded Gaza relentlessly from the air, land and sea since October 7 when Hamas militants stormed into southern Israel killing around 1,200 people, mostly civilians, and kidnapping some 240 others, according to Israeli authorities.

Since then, health officials in Hamas-run Gaza say Israel's relentless campaign has killed at least 13,000 people, also mostly civilians.

The operation in Al-Shifa has drawn widespread international condemnation, with the World Health Organization describing the hospital as a "death zone" after sending in a team to visit the facility on Saturday.

The visit came after hundreds fled the hospital following what Al-Shifa's director said were Israeli army orders for it to be emptied, with an AFP journalist seeing crowds of sick, injured and displaced people fleeing on foot.