Israeli aircraft struck repeatedly at Palestinian rocket squads in northern Gaza on Wednesday, and in one botched attack killed a 12-year-old boy, his father and uncle.

Islamic militants, enraged by the deaths of 19 Palestinians a day earlier, barraged southern Israel with rocket and mortar fire. Among the dead Tuesday was the son of Gaza's most powerful leader, Mahmoud Zahar, and the Islamic group that controls the territory vowed to retaliate for the bloodshed.

The intensified clashes came on a day when a hawkish Israeli political faction weakened Israeli Prime Minister Ehud Olmert's political base by pulling out of his government, while potentially freeing his hand for greater concessions to the Palestinians.

"Negotiations on the basis of land for peace is a fatal mistake," Avigdor Lieberman told a news conference in Jerusalem in announcing the withdrawal of his hardline Yisrael Beiteinu Party.

The Palestinian civilians were killed in an Israeli air attack on a pickup truck east of Gaza City. The Popular Resistance Committees, a small, Hamas-allied faction, said the apparent target was its chief rocket maker, who was driving in the area in a similar vehicle at the time.

Relatives identified the dead as 12-year-old Amir Yazagi, his father Mohammed and uncle Amr, said Dr. Moaiya Hassanain of the Gaza health ministry. Their bodies were so mutilated that it was hard to identify them, medics said. Originally the medics thought the two men were militants because one was wearing a military-style jacket.

Maj. Avital Leibovich, an Israeli military spokeswoman, acknowledged that the Yazagi family's vehicle was "unintentionally hit."

Civilians regrettably are hurt when militants operate in civilian environments, Leibovich said. "It is important to me to stress that we have no intention whatsoever to hit or hurt uninvolved civilians," she said.

Hamas spokesman Taher Nunu called the strike "a new crime," saying Israel was "killing more and more of our innocent people and our freedom fighters."

In border communities in southern Israel, the siren warning of rocket attacks rang repeatedly Wednesday morning as 21 rockets and mortars were fired, the Israeli military said. No serious injuries or damage were reported.

Residents of Sderot, a town of 20,000 that is a frequent target, stayed off the streets as the sirens blared.

The Hamas government called a three-day mourning period for the 19 Palestinians killed on Tuesday, and a general strike took hold across the territory.

Palestinian flags were lowered to half-staff, verses from the Quran, the Muslim holy book, poured forth from mosque loudspeakers, and government offices, banks and shops were shuttered. Government offices and shops were also closed in the West Bank, which is controlled by Hamas' bitter rival, the moderate Palestinian President Mahmoud Abbas. Leading officials from Abbas' government called Zahar to express condolences.

On Wednesday, Iranian President Mahmoud Ahmadinejad called the head of Gaza's Hamas government, Ismail Haniyeh, to express his condolences for the death of Zahar's son, Nunu said.

The hardline Iranian leader accused U.S. President George W. Bush of giving Israel his approval for military operations in Gaza during a peacekeeping mission to the region last week.

"Without the green light of the criminal Bush, yesterday's massacre would not have occurred," Ahmadinejad said, according to Nunu.

In West Bank violence, Israeli troops killed the Islamic Jihad group's top commander in a predawn raid on the village of Qabatiya south of Jenin, the group said. Walid Obeidi was wanted by Israel for years, Islamic Jihad said, vowing revenge.

The Israeli army said troops tried to arrest Obeidi, but he resisted and was killed in an exchange of fire.

The intensified clashes with Palestinian militants are complicating recently renewed peace talks that are already fraught with tension over Israeli construction in disputed territory and Palestinian violence in both Gaza and the West Bank.

The defection of Lieberman's Yisrael Beiteinu Party did not deliver a fatal blow to Olmert's government, which still commands 67 of parliament's 120 seats. But with other factions also making noises about quitting, and a potentially damning report on the Lebanon war forthcoming, the assault on Olmert's leadership threatened to snowball.

Lieberman bolted the government after negotiators on Monday began tackling the core issues of Israel's decades-old conflict with the Palestinians — final borders, sovereignty over disputed Jerusalem, and Palestinian refugees who lost homes in Israel during the war that broke out following the Jewish state's creation in 1948.

"If we pull back to the 1967 borders, everyone should ask himself, what will happen the following day?" Lieberman asked. "Will the conflict stop, will the terror stop? Nothing will change."

Olmert's office released a statement saying the prime minister was determined to pursue peacemaking.

"There is no substitute for serious negotiations with a goal of achieving peace," the statement said. "That is the order of the hour."

In related news, Israeli forces evacuated two makeshift settlement outposts in the West Bank, an incremental move against a major obstacle to Israeli-Palestinian peace talks. Five people at the Harchivi outpost fled as police arrived, but some of the 20 protesters at the Shvut Ami outpost had to be carried away.

Both outposts have been evacuated in the past, said the anti-settlement watchdog Peace Now.

Bush told Israel during his trip last week to get rid of the roughly two dozen outposts it has promised to dismantle.