The Israel Defense Forces on Sunday released detailed information on a five-story building it destroyed in one of its retaliatory airstrikes in the Gaza Strip the previous day, in response to repeated mortar and rocket attacks from the coastal enclave that have pummeled southern Israel.

According to the army, the building was being used as a training facility by Hamas and sat atop a tunnel that fed into a “massive” underground network.

On Saturday afternoon, following dozens of attacks by Hamas and other terrorist groups, Israeli Air Force jets dropped a number of bombs on a five-story building in the Shati refugee camp in the northern Gaza Strip.

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The initial bombs were meant to scare away those in the area, before the subsequent projectiles razed the building to the ground, the army said.

In addition, the IDF distributed “before and after” aerial photographs of another target, a Hamas battalion headquarters in the city of Beit Lahiya, also in northern Gaza, which was hit earlier in the day.

The photographs show that the facility’s buildings were reduced to rubble in the airstrike. The military said the compound was made up of: a training ground, a weapons manunfacturing and storage facility, two commanders’ offices and a logistics shed.

Beginning at 1:30 a.m. on Saturday, over 174 rockets and mortar shells were fired at southern Israel from Gaza. The majority of them, well over 100, landed in open fields. Over 30 of them were intercepted by the Iron Dome missile defense system. A number landed within the fences of communities in southern Israel.

Three Israelis were wounded when rockets hit a home and a synagogue in the border town of Sderot. They were evacuated to the hospital in moderate condition.

A number of other projectiles that landed inside communities in the Eshkol region of southern Israel caused light damage to buildings and infrastructure. In some communities, the mortar shells knocked down power lines, causing temporary outages, a spokesperson for the regional council said.

In response, the IDF launched its largest bombing campaign against Hamas targets in the Strip since the 2014 Gaza war, hitting dozens of targets, the military said.

In addition to Hamas’s five-story urban combat training facility and the battalion headquarters, the IDF destroyed two border-crossing attack tunnels, Hamas naval facilities and dozens of other military compounds, the army said.

According to the Hamas-run Gaza health ministry, two Palestinians teenagers were killed in the Israeli strike on the building in Shati and 25 others were injured by shrapnel.

The Hamas-run health ministry identified the dead as 15-year-old Amir al-Nimra and 16-year-old Louay Kahil.

The largely abandoned building was known as the National Palestinian Library and was located next door to the Sheikh Zayed mosque, which sustained light damage as a result of the Israeli airstrike.

“The building’s five floors were supposed to be used for residents of the Strip, for public and government services or at least for housing. Instead, for the past few years, the large building has been used as a training facility for Hamas’s fighting battalions for urban warfare, exercises in conquering buildings and recently as a facility for surviving inside tunnels — thank to an attack tunnel that was dug underneath the building,” the army said in a statement.

According to the IDF, the tunnel underneath the building was connected to a “massive network of subterranean tunnels” dug by Hamas, which rules the Gaza Strip.

“Hamas continues to use civilian infrastructure for military purposes and in that way it endangers the lives of the civilians under its protection,” the army said.

“A weapons manufacturing site and storage facilities housing various types of weapons, including Hamas’ naval capabilities, were [also] struck,” the IDF said.

The air force also attacked two mortars, which had been used to launch shells into southern Israel.

The IDF spokesman said the aim of its airstrikes was to “restore a sense of security” and that the military would “respond as necessary” to a wide range of scenarios.

The Israeli military held Hamas, which rules Gaza, as responsible for Saturday’s flare-up.

“The Hamas terror group is responsible for everything that goes on in and from the Gaza Strip and it will bear the responsibility for the situation,” the IDF said.

The military threatened that it was prepared to take more forceful action “based on situational assessments and operational need.”

On Saturday night, Hamas and the Palestinian Islamic Jihad terrorist groups announced that they had agreed to a ceasefire with Israel, following talks with Egypt and other international bodies.

“The regional and international mediation has led to an end of the current escalation between the resistance and the occupation forces,” Hamas said.

A senior Israeli defense official would not directly comment on the reported ceasefire, but said: “Facts on the ground will determine our continued response.”

On Saturday night, Prime Minister Benjamin Netanyahu vowed that Israel would step up its strikes on Gaza until the Palestinian terror groups halt all violence coming out of the coastal enclave.

“The IDF hit Hamas with the harshest blow since Operation Protective Edge and we will intensify our reaction as much as necessary,” Netanyahu said Saturday evening, after the IDF attacked dozens of targets in the Strip on Saturday, in the most extensive daytime assault since the 2014 conflict.

“If Hamas does not understand the message today, it will understand tomorrow,” Netanyahu said in a video statement.

Over the last few months Palestinians in Gaza have sent over thousands of kites and balloons attached to incendiary devices that have set hundreds of fires in farm lands and nature reserves along the border with Gaza, destroying tens of thousands of acres.

Earlier in the day the IDF said it had three main aims: Stopping the incendiary kites and balloons, the rocket attacks, and the weekly protests along the Gaza border.

The army said it held Hamas responsible for all violence emanating from Gaza, which the terror group has ruled since 2007.