The pre-dawn salvo on Ashkelon, about 12 km (7 miles) north of Gaza, was unusual given the relative restraint of Palestinians since last November's war between the coastal enclave's Hamas and the Jewish state.
There was no immediate claim of responsibility for the launches.
Hamas has tried to mend ties with Egypt while appealing to rival Palestinian President Mahmoud Abbas, whose administration holds sway in the occupied West Bank, to abandon U.S.-sponsored peace talks with Israel.
It has also signalled readiness to fight again, taking responsibility last week for a tunnel which the Israelis discovered running into their territory from Gaza and said was likely intended for kidnapping soldiers or setting off underground explosives.
The Israeli military spokeswoman said a mortar bomb was fired across the border from Gaza on Sunday, but caused no damage or casualties.