Israeli aircraft unleashed a barrage of missiles early Wednesday and fired artillery into the Gaza Strip for the first time, pushing forward with an offensive despite a pledge by Islamic militants to halt their recent rocket attacks against Israel.

Renewed fighting that entered its fifth day Wednesday has compounded Israeli Prime Minister Ariel Sharon's (search) political problems. The violence had been expected to harm Sharon's chances in a vote Monday in the ruling Likud Party (search), where hard-liners hoped to punish him for the Gaza withdrawal.

Sharon's narrowly won that challenge, but an advisor said Tuesday he still may bolt the party if it refuses to support his political program.

The Israeli airstrikes early Wednesday knocked out power throughout nearly all of Gaza City. The army said it targeted three buildings used for "terror activity" by the ruling Fatah Party (search), as well as two smaller armed groups. Palestinian security officials said there were no injuries.

Israeli Defense Minister Shaul Mofaz said the army would attack Palestinian militants relentlessly to force them to stop firing rockets at Israeli towns.

The fifth straight day of airstrikes came hours after Islamic Jihad militants on Tuesday declared a halt to their recent rocket attacks, and armed Palestinian groups pledged to honor a tattered cease-fire, seeking to end the Israeli offensive.

Tensions were further inflamed when Hamas militants released a video showing a bound and blindfolded Israeli businessman whom they kidnapped and later killed. The kidnapping appeared to signal a new tactic in the militants' fight against Israel.

Israel launched its offensive early Saturday in response to rocket fire from Gaza. It has carried out numerous airstrikes in Gaza and arrested hundreds of Palestinians in the West Bank, saying the operation will continue until the rocket attacks stop.

Israeli security officials welcomed the cease-fire declaration but said they wanted to see concrete results before halting the offensive. As the militants were meeting, a rocket landed in the southern Israeli town of Sderot, causing no damage or injuries, the army said. No one claimed responsibility for the attack.

Late Tuesday, the army fired live artillery shells into northern Gaza for the first time in what it said was a response to Palestinian rocket attacks. The shells landed in an open area that the army said was used to fire rockets. No casualties were reported.

Israel had previously refrained from retaliating against Palestinian attacks with artillery because it is more imprecise than missiles and could cause many casualties in densely populated Gaza.

The new violence started after a blast Friday at a Hamas rally in Gaza's Jebaliya refugee camp killed 21 people, including a 7-year-old boy who died of his wounds Tuesday.

Hamas said Israeli aircraft had fired missiles into the crowd, which Israel denied. The Palestinian Authority has said the blast was caused when Hamas militants mishandled explosives.

In the vote Monday, widely seen as a referendum on Sharon's leadership and the Gaza withdrawal, the prime minister prevailed with a slim margin. His allies had said that if he lost, he might leave Likud, call early elections and run as head of a new centrist party.

Sharon's political adviser, Lior Horev, said Tuesday that Sharon might still bolt the divided party if it refuses to back his major policies. "Either the party stands behind him, or he has to choose a different way in order to push forward his agenda," Horev said.

Sharon's main rival, Benjamin Netanyahu, said the fight was not over and insisted he would prevail in party primaries next year.