Israel Defense Forces troops exposed a 180-foot tunnel 32 feet deep underneath the Shifa Hospital, the IDF and Israel Securities Authority (ISA) said in a joint statement Sunday.

"A deep staircase leads to the entrance of the tunnel shaft, which consists of various defense means including a blast-proof door and a firing hole," the statement continues. "This type of door is used by the Hamas terrorist organization to block Israeli forces from entering the command centers and the underground assets belonging to Hamas. The tunnel shaft was uncovered in the area of the hospital underneath a shed alongside a vehicle containing numerous weapons including RPGs, explosives and Kalashnikov rifles.

"IDF and ISA forces are continuing to uncover the route of the tunnel," the statement adds.

The IDF said it expects to find more tunnels and underground facilities, and that the operation is ongoing and will take some time. About 300 people remain in the hospital, according to the IDF.

-ABC News' Yael Benaya

The World Health Organization confirmed earlier claims by the Palestinian Health Ministry that 31 "very sick" babies were safely transported from Al-Shifa Hospital to another hospital in southern Gaza on Sunday.

The premature infants were evacuated to the Emirates Hospital in Rafah, WHO officials said.

The babies were "clinging to life," the United Nations International Children's Emergency Fund (UNICEF) said in a statement, and their "condition was rapidly deteriorating."

The babies are expected to be transferred to another hospital in Egypt, said Dr. Ashraf al Qwadwa, a spokesperson for the Palestinian Health Ministry. He said nine premature babies have died since the start of a fuel crisis in Gaza prompted by the ongoing conflict.

Six health workers at the Al Shifa Hospital and 10 family members of staff there were also evacuated on Sunday along with the babies, according to the WHO statement, adding that further missions are planned to "urgently transport" the remaining patients and health staff out of the hospital.

-ABC News' Nasser Atta

The war, which has now moved into its second stage, according to Israel, has passed the one-month mark.

In Israel, at least 1,200 people have been killed and 6,900 others have been injured since Oct. 7, according to Israeli officials. In the neighboring Gaza Strip, at least 13,000 people have been killed and over 30,000 have been injured, according to the Hamas-run Gaza Health Ministry.

Aid workers and officials fear that Israel's call for an evacuation of the northern part of Gaza is precipitating a humanitarian disaster as electricity and other supplies have been cut off in preparation for what appears to be an imminent ground offensive.

Humanitarian groups have urged Israel to call off the evacuation and agree to a cease-fire, even as the country has asserted a right to defend itself -- a right the United States endorses.