At least six Palestinians, including three from the same family, have been killed in Israeli attacks in Gaza and the West Bank.
Hamas responded to Wednesday's deaths by launching more than three dozen rockets into Israeli territory and demanding an end to peace talks.
The killings came as George Bush, the US president, expressed optimism that peace would come to the region as he wrapped up his Middle East tour.
They also came a day after 18 other Palestinians were killed, in an attack Mahmoud Abbas, the Palestinian president, termed a "massacre" against his people.
As Palestinians held a general strike over Tuesday's killings, an aircraft missile aimed at fighters from the Palestinian armed group, Islamic Jihad, in Gaza City hit the wrong car and killed a 13-year-old boy, his father and an uncle.
Medics said the bodies were so mutilated that it was hard to identify them.
Major Avital Leibovich, an Israeli military spokeswoman, said the civilians' car destroyed in the earlier attack had been "unintentionally hit".
Israeli officials said another vehicle had been the intended target of the attack.
Islamic Jihad said that one of its cars was hit in the attack, but that its fighters had escaped.
A second Israeli air strike on a car killed two Palestinians in the central Gaza Strip, the ruling Palestinian faction, Hamas, said. A third raid, soon after, left no casualties.
Al Jazeera's Jacky Rowland, reporting from Gaza, said residents were constantly looking up in the sky, fearful of more attacks.
West Bank firefight
In the West Bank town of Qabatiya, near the city of Jenin, Israeli forces killed Walid al-Ubaidi, the leader of Islamic Jihad's al-Quds Brigades, in a gunfight and wounded and arrested two other fighters.
An Israeli army spokesman said al-Ubaidi was killed during an exchange of fire with troops who came to arrest him.
The al-Quds Brigades, the military wing of the Islamic Jihad group, confirmed his death.
Gaza was at a standstill on Wednesday as Palestinians held a general strike in mourning for 18 people killed in an Israeli raid the day before, the enclave's bloodiest day since Hamas seized control of the territory in June last year.
In a rare show of unity, both Hamas and the Fatah faction led by Abbas declared three days of mourning for those killed in Tuesday's raids, ordering the closure of government offices, businesses, shops and schools.
"The strike shows that we and Gaza are one people in the face of Israeli massacres," said Majdi Maraqa, a shop owner in the West Bank city of Ramallah.
Abbas had on Tuesday described the killings as "a massacre" and "a slaughter against the Palestinian people".
"Our people cannot keep silent over these massacres. These massacres cannot bring peace," he had said.
Abbas said the Gaza operation on Tuesday, had severely damaged the peace efforts relaunched by Bush in the US city of Annapolis less than two months ago.
Tuesday's attack also killed Hussam al-Zahar, the son of Mahmoud al-Zahar, a Hamas leader and the foreign minister in the unity government that Abbas sacked when Hams took over Gaza.
That killing prompted the first official contact between Abbas and Hamas since June.
A Hamas spokesman said Abbas called al-Zahar to offer his condolences.
He said the "conversation was very friendly and the two leaders spoke at length about the current political situation and they both stressed the unity of the Palestinian people regardless of the differences".
Hamas said it was Bush's presence in the region that was spurring the violence.
Khaled Meshaal, the group's exiled political chief, said he held Bush and Israel accountable for the deaths of the Palestinians killed during Israel's raids on the Gaza Strip.
"We would like to tell George Bush that this is the real terrorism. Arabs and Muslims are not the terrorists," Meshaal said at a press conference in the Syrian capital, Damascus, on Wednesday.
He said: "An Israeli official said that Bush gave Olmert the green light to launch a full scale military incursion on Gaza.
"Therefore I hold the American administration responsible for what happened yesterday in Gaza. Bush is a man of war and crime, not peace and security."