The armed wing of Hamas, Izz al-Din al-Qassam, took responsibility for a barrage of rocket fire on Israel on Monday night, claiming to have shot some 100 rockets within a few minutes at the South.Heavy rocket barrages slammed into southern Israel on Monday evening, triggering air raid sirens across wide areas, including Rehovot and Beit Shemesh.Seven long-range Palestinians rockets were intercepted over Ashdod, and five over Netivot. Some 30 rockets were fired from Gaza in the Monday night barrage. Sixteen projectiles also landed in open areas near the southern city of Beersheba, according to the IDF Spokesperson's Unit.A total of 80 rockets slammed into southern Israel throughout the day and evening.Hamas has vowed revenge for what it claims was Israel's deadliest attacks in which seven Palestinian militants died on Monday.A senior military source said the seven Hamas members did not die as a result of an Israeli air strike. The source said they died as a result of explosives they planted in an underground assault tunnel aimed at an IDF target. The tunnel was found by the army a few days ago. "Last night, for reasons that are unclear, Hamas decided to handle the explosives. They handled bombs that were in the tunnel, and were ready to go off against military targets. The explosives went off, leading to seven casualties," said the source. Hamas spokesman Sami Abu Zuhri accused Israel of committing a "grave escalation" in violence and threatened to retaliate, saying Israel would "pay the price".The rocket attacks came shortly after a senior military source said the IDF is taking “significant steps” to prepare itself for a stepped up campaign of attacks against Hamas in Gaza. He spoke as Palestinian projectile fire from the Strip continued unabated.The Home Front Command has instructed the municipalities of Ashkelon, Ashdod, Beersheba and Gaza border communities to open public bomb shelters in anticipation of long-range Hamas rocket attacks.Air raid sirens were also heard on Monday in the outskirts of Jerusalem and Tel Aviv, but an Israeli police spokesman said they were false alarms.Reuters contributed to this report.