Earlier, an Israeli air attack on a car in the northern Gaza Strip killed a civilian and two Palestinian fighters, identified by Hamas as members of the Islamic Jihad group.
The Israeli army confirmed the air raid. The civilian, a woman, had been riding on a donkey cart nearby.
The other two Palestinians killed in an earlier air raid were identified by Hamas as Raed Abu al-Foul, a leader of the Popular Resistance Committees (PRC), and his wife.
A PRC spokesman, in response to the killings, said: "It is too late for the enemy to try to stop our rockets, we have hundreds of rockets and they are ready to be fired at the Zionist settlements near Gaza Strip.
"One martyr goes to heaven and thousands rise to fire more rockets and to fight the Zionist entity."
Palestinian fighters have fired a steady stream of rockets into Israel.
Up to 17 Qassam rockets fired from Gaza reportedly landed on the Israeli town of Sderot on Thursday.
Though no people have died in the Palestinian rocket attacks, one house was severely damaged.
Olmert on Thursday vowed to force Palestinian fighters halt their rocket attacks.
"A war is going on in the south, every day, every night," he said.
"We cannot and will not tolerate this increasing fire at Israeli citizens ... so we will continue to operate, with wisdom and daring, with the maximum precision that will enable us to hit those who want to attack us."
Jacky Rowland, Al Jazeera's correspondent in Gaza, said that the Israeli prime minister's words would not come us a surprise for people in Gaza.
"They feel it's been war for a very long time now."
At least 25 people have been killed in Gaza over the past three days, including the son of Mahmoud al-Zahar, a senior Hamas leader.
Hamas says its fighters launched around 80 rockets and mortars towards Israel on Wednesday.
An Ecuadorean volunteer working on an Israeli farm was killed by a Palestinian sniper the day before.
In a rare show of unity, both Hamas and its rival Fatah, led by Mahmoud Abbas, the Palestinian president, declared three days of mourning when 18 people were killed on Tuesday in Gaza.
Abbas also reportedly called al-Zahar to offer condolences for his son's death and described the killings as "a massacre" and "a slaughter against the Palestinian people".
Nabil Amer, an adviser to Abbas, held Israel responsible "for every drop of blood that is shed in the Gaza Strip".
Meanwhile, a senior Hamas official hailed the renewed contact between the rival Palestinian factions, saying now is the time for the two sides to return to the negotiating table.