The bombardment was confirmed by the Israeli army which said militants had launched "a few dozen rockets" within a short period of time.
At least four were intercepted over Netivot by the Iron Dome anti-missile system, while another 16 struck the area around the southern city of Beersheva, some 40 kilometres from Gaza, and which is home to 200,000 people, the army said.
Media reports put the number of rockets at around 40, but there were no reports of casualties.
"Al-Qassam fired dozens of rockets on Netivot and Ashkelon, Ashdod and Ofakim in response to the Zionist aggression," a statement said.
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The rocket fire came several hours after Israel staged around 16 air strikes on targets across Gaza, following a night in which warplanes had attacked 14 targets, killing at least three militants.
Another five Hamas militants died and one was left in critical condition when a tunnel collapsed near the southern city of Rafah, with the movement blaming it on an Israeli air strike.
There was no immediate word on casualties or damage in Israel from the latest rocket attacks.