Earlier one man died and several were hurt when Israeli helicopter gunships fired on them after they had launched a rocket into Israel, a Palestinian hospital official said.
Hit by missile
Officials said other men died when a base used by the armed wing of Hamas in the east of the territory was hit by a missile from an F-16 fighter jet overnight.
Two missiles were also fired at a security forces building in central Gaza but no casualties were immediately reported.The attacks came after four Palestinians had been killed in a series of air strikes on Thursday.
The Israeli military confirmed it had carried out the attack on the building it said was used by the Izz al-Din al-Qassam Brigades as a meeting place in advance of attacks on Israel.
An Israeli official also said that the building was believed to have a tunnel for smuggling weapons.
Four other members of the group were injured when the building was hit.Rocket attacks
On Thursday, an Executive Force compound, another base, three cars and a training camp were destroyed in response to Palestinian rocket attacks on the Israeli town of Sderot.
Israel says that more than 100 rockets and mortars have been launched from the Gaza Strip this week.
"We will see sustained and vibrant measures to end the attacks to end the rocket attacks and remove the threat to southern Israel," David Baker, an Israeli government spokesman, said on Friday.
Ehud Olmert, the Israeli prime minister, vowed to stand firm during a visit to Sderot late on Thursday.
"I am handling this crisis in order to remove this threat as much as possible," he told residents, according to a statement released by his office.
Israeli tanks and armoured vehicles also entered the Gaza Strip, the coastal enclave from which Israel withdrew in 2005. A spokesman said it was a "small force" on a "defensive operation".
"The Palestinian residents of Gaza City are trapped between Palestinian internal fighting on the one hand and Israeli helicopters and aircraft overhead on the other, threatening imminent strikes," Nour Odeh, Al Jazeera's correspondent in Gaza, said.
Abu Dhabi Television, meanwhile, announced that Hamas fighters had kidnapped Abdel Salam Abu Askar, its Gaza bureau chief. Hamas denied the charge.
Sporadic fighting between loyalists from the Fatah and Hamas political factions also broke out on Friday despite a ceasefire agreed late on Wednesday.
|Palestinians try to move a minibus hit by|
an Israeli missile in Gaza on Friday [AFP]
Rocket-propelled grenades were fired at the Hamas-dominated Islamic University. Hamas officials blamed the Presidential Guard of Mahmoud Abbas but a spokesman for the Guard accused Hamas of using the university as a base for attacks on nearby police stations.
Ismail Haniya, the prime minister, has urged Palestinians to unite against "Israeli aggression" and cease internal fighting.
All members of the security services should abide by the instructions of the political leadership and return to their positions and bases, and also all gunmen should pull out of the streets," he said.
After the Israeli air strikes on Thursday, Hamas officials accused their Fatah rivals of colluding with the Israeli military.
"Israeli ocupation forces are bombarding Hamas headquarters and the Presidential Guard is bombarding the Islamic University," Fawzi Barhum, Hamas movement spokesman, said.
"We wonder where the Presidential Guard get their orders."