A second Israeli air strike on a car killed two Palestinians in the central Gaza Strip, the ruling Hamas faction said.
An Israeli military spokeswoman confirmed there had been an air strike, saying it targeted a vehicle transporting munitions.
A third raid, soon after, left no casualties.
Living in fear
Major Avital Leibovich, an Israeli military spokeswoman, said the civilian's car destroyed in the earlier incident had been "unintentionally hit".
Jacky Rowland, reporting for Al Jazeera from Gaza, said residents were fearful of another attack.
She said: "Just standing out in the street in Gaza City, people are looking up to the sky ... People are expecting more [attacks].
"The Isrealis say they are targetting fighters, but as we've seen today, ordinary people can often be those who end up paying with their lives for this violence."
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.
West Bank violence
Israeli forces shot dead Walid al-Ubaidi, the leader of Palestinian group Islamic Jihad's al-Quds Brigades, in the West Bank on Wednesday.
Israeli troops operating in the northern West Bank town of Qabatiya, near the city of Jenin, killed al-Obeidi in a gunfight and wounded and arrested two other fighters, witnesses said.
An Israeli army spokesman said al-Obeidi 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.
Palestinian fighters fired at least 21 rockets toward Israel on Wednesday but no injuries or serious damage were reported, the Israeli military said.
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 Strip's bloodiest day since Hamas seized control of the territory in June last year.
To mark three days of mourning, flags flew at half mast, Gaza streets were empty and parents kept their children home from school.
In solidarity with Gaza, the West Bank also held a general strike, with many shops shut in the city of Ramallah, home to the Palestinian Authority led by Mahmoud Abbas, the Palestinian president.
"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 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 said.
Israel's incursion came a week after George Bush, the US president, said during a visit to the region he wanted a peace deal between the Israelis and the Palestinians by the end of his presidential term at the end of 2008.
Hamas officials blamed Bush's presence in the region for the violence.
Abbas said the Gaza operation, in which Hussam al-Zahar - the son of Mahmoud al-Zahar, a former Hamas foreign minister - was also killed, had severely damaged the peace efforts relaunched by Bush in the US city of Annapolis less than two months ago.
Israel said they were engaging in an operation "against terror threats".
Khaled Meshaal, the exiled political chief of Hamas, said he held Bush and Israel accountable for the deaths of the Palestinians killed during Israel's raids on the Gaza Strip.
As a press conference in the Syrian capital, Damascus, Meshaal said: "We would like to tell George Bush that this is the real terrorism. Arabs and Muslims are not the terrorists.
"An Israeli official said that Bush gave Olmert the green light to launch a full scale military incursion on Gaza," Meshaal said.
"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."
At least 115 Palestinians have been killed by Israeli fire in Gaza, according to a tally by the AFP news agency, since the Annapolis Conference on finding a two-state solution to the Palestinian-Israeli conflict was held in November.