Gaza militants fired dozens of shells into Israel, reportedly wounding at least two people, while Palestinians said five had been hurt in Israeli air strikes.
The armed wing of the radical Hamas movement governing the Palestinian territory said it had mounted the attack, three days after an Israeli air strike killed two of its members in the Gaza Strip.
Israel said 49 mortar bombs fell on or around Israeli communities near the border with the Gaza Strip in the biggest violation yet of an informal truce militants declared in January.
Army radio said only minor damage was caused but an Israeli military spokesman said two civilians were lightly wounded when three projectiles fell on Pithat Shalom in the Negev desert.
Two more had hit Eshkol, further north, causing neither damage nor casualties, the spokesman said.
Israeli radio said residents were advised to take shelter until further notice.
The Palestinian Ezzedeen Al-Qassam Brigades said they had targeted the border crossing points of Sofa, Kessufim and Nahal Oz.