Hamas and Fatah gunmen exchanged automatic weapons fire Friday at a Gaza City university and Israeli war planes pummeled Hamas targets for a third straight day, killing eight people in response to the Islamic group's rocket attacks on southern Israel.

The sound of gunfire and explosions rang out in the Gaza Strip as masked gunmen took up positions at roadblocks and rooftops — defying mediators who worked furiously to get the sides to withdraw and halt their fire.

Against the backdrop of the Palestinian infighting that threatened all-out civil war, Israeli airstrikes added an extra element of violence and uncertainty. Still, a senior army official said Israel had no immediate plans for a major ground offensive to halt rocket fire.

The Israeli army said about 90 rockets have hit southern Israel since Wednesday. At least 13 fell on Friday, including one that wounded three Israelis and another that wounded a man in the southern town of Sderot.

Six days of fighting between Hamas and the Fatah movement of Palestinian President Mahmoud Abbas have claimed 47 lives and all but destroyed a two-month-old power-sharing deal between the two groups. An additional 20 Palestinians have died in the Israeli airstrikes.

Israel launched five new airstrikes on Friday alone. One east of Gaza City killed five Palestinians, at least three of them Hamas militants, and wounded six people, Hamas and local doctors said. The military said the target was a Hamas headquarters building.

Four other strikes followed, the last of which targeted a minivan belonging to a Hamas fighter in northern Gaza City, killing three people and wounding 12, Palestinian hospital officials said.

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An afternoon hit on a Hamas military building near the central Gaza town of Deir al Balah. No casualties were reported, most likely because Hamas had ordered its people to evacuate possible Israeli targets. But Hamas suffered further damage to its infrastructure.

Gunshots and rocket-propelled grenades flew outside the Islamic University, a Hamas stronghold, in two bursts of fighting on Friday. Hamas fighters in control of the university battled Fatah forces who had taken up positions in the nearby Foreign Ministry building.

Grenades hit the office of university President Kamelen Shaath, who appealed for an immediate halt to the violence.

"Universities must be outside the circle of violence and I appeal to the president (Abbas) and all the wise people on both sides to try and spare the university the agony of this fight," he said.

One person was wounded in fighting at the school.

Elsewhere in the city, a 40-year-old Palestinian fisherman was shot in the head by a sniper. It was not clear which side fired the deadly shot.

Street battles were down from their height two days ago, but the latest truce worked out Thursday enjoyed no more success than a series of previous cease-fires. That raised the question of who was in charge — and it appeared the political leaders of both Hamas and Fatah had lost control of their gunmen.

"Our retaliation for (Fatah's) crimes is going to be beyond their imagination," Abu Obeida, spokesman for Hamas's military wing, told The Associated Press.

Gen. Jamal Kayed, Fatah's security commander in Gaza, said his group had already begun implementing the cease-fire but said Hamas was not willing to follow suit.

By most accounts, Hamas's performance in the latest round of internal fighting has been superior to Fatah's, with greater discipline and more motivated fighters.

Although Israel said it wasn't taking sides, its airstrikes made it harder for Hamas gunmen to move around, and Hamas used that fact to argue that Fatah and Israel were in cahoots. Hamas TV on Friday named three Fatah security chiefs who it said were in secret contact with "foreign" security personnel to exchange information on Palestinian militant groups.

"They are deep into treason, and we will deal with them accordingly," the broadcast said. The TV did not specify which foreigners, but Fatah forces affiliated with Abbas have received advice and training from the U.S.

Earlier in the week, some 500 Fatah security forces trained in Egypt under a U.S.-brokered deal returned to Gaza, passing through the border with Israel's permission. While Israel and the U.S. have made no secret of their desire to bolster Fatah at the expense of Hamas, Israeli policymakers also want to avoid uniting Palestinian factions into a common front against Israel.

Washington lists Hamas, which has killed more than 250 Israelis in suicide and other attacks, as a terror group. Hamas' parliamentary election sweep last year provoked punishing sanctions against the Palestinian government that have remained in place despite the formation of a national unity government in March.

International donors are demanding that Hamas renounce violence and recognize Israel's right to exist as a condition for restoring aid. Hamas' decision to resume violence in the form of rocket attacks on Israel is likely linked to the unity government's failure to lift the boycott.

Israeli media reported that between 2,500 to 3,000 of Sderot's 23,000 residents have fled the rocket-battered city in recent days, some leaving on buses organized by the government, and others taking advantage of a Russian-Israeli tycoon's offer to stay in hotels in Tel Aviv, Jerusalem and Be'er Sheva at his expense.

Israeli Foreign Minister Tzipi Livni showed members of the diplomatic corps video of a Sderot school damaged by rocket fire. "For too long the international community took the situation in the south of Israel as acceptable, as part of life in Israel, and it's not," she told Tel-Aviv based envoys.

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