The Al-Aqsa Martyrs Brigades, an armed offshoot of the ruling Fatah faction, said it had fired two missiles at dawn into southern Israel in response to the killing of two Palestinians on Saturday.
Saturday marked one of the most violent days in Gaza since Israel pulled its troops out of the territory in September.
The Israeli military also reported that makeshift rockets landed in the
village of Shuva in the Negev desert, damaging a house.
There was no immediate claim of responsibility.
An army spokeswoman later said five rockets had fallen.
Israeli forces retaliated with artillery fire, concentrating on an area said to be the source of the attacks in northern Gaza.
The new attacks did not cause any casualties.
But they looked set to trigger more retaliation, after Israel’s defence minister and army chief of staff both pledged to match fire with fire.
"One of these attacks targeted an office organizing terrorist activities and three others targeted a sector in the north of the region from where Qassam rockets have been fired at Israel," the Israeli army spokeswoman told AFP.
"These were reprisals after the Palestinians fired Qassam rockets."
An AFP correspondent in Gaza said at least two helicopter gunships took part in the raids.
Three makeshift Qassam rockets hit Israeli territory on Saturday without causing any casualties.
A military source said the rockets fell near the Carmilla kibbutz, south of the town of Ashkelon, and on the village of Nativ Ha Assera.
Two others landed inside the Gaza Strip.
Soldiers close to the border responded on Saturday with a sustained barrage of artillery fire from Israel into an uninhabited area of Gaza.
Defence Minister Shaul Mofaz has reiterated that Israel would not sit with its hands crossed in the wake of attacks by militants.
"I have ordered the army to take immediate offensive measures in reaction to the rocket and mortar firing ... If calm does not reign in Israel, it will not reign in Gaza," he told public radio.
The army chief of staff, General Dan Halutz, also underlined that the "bombing will not stop until attacks stop."
"Do not test our abilities ... because it will create a lot of destruction in Gaza," Halutz said to journalists in Tel Aviv.
Mofaz however stressed that the levels of violence were lower than before Israel's exit from Gaza.
"The number of terrorist attacks have fallen since the pullout was completed while our military responses have become more effective, even if the terrorists continue to carry out attacks from time to time," he added.
Israeli soldiers shot dead two Palestinians in Gaza early Saturday in
separate incidents.
The dead include a fisherman who was killed off Rafah in the south of the Gaza Strip by shots from an Israeli naval patrol boat.
Elsewhere in Gaza on Saturday, five Palestinians including a policeman were killed and 10 more wounded Saturday in a fierce gunbattle between members of rival clans in the northern town of Beit Hanun.