Among the Palestinian dead was a news cameraman.
The surge in violence came after a relatively quiet month and threatened to unravel an Egyptian effort to mediate a cease-fire between Israel and Hamas.
Wednesday's death toll was the highest since an Israeli military offensive in early March that killed more than 120 Gazans, including dozens of civilians. Since then, Israel and Hamas appeared to be honoring an informal truce, though punctuated with Palestinian rocket attacks, some Israeli airstrikes and border skirmishes.
That changed dramatically Wednesday with no apparent trigger, indicating the relative calm was more coincidence than plan.
In the day's deadliest attack, an Israeli helicopter fired four missiles at targets near the Bureij refugee camp in central Gaza, witnesses said. At least 12 Palestinians, including five children aged 12-15, were killed, said Dr. Moaiya Hassanain of the Palestinian Health Ministry.
Reuters cameraman Fadel Shana was killed while filming Israeli tank movements, apparently in an airstrike in the same area. Two bystanders also died.
Other cameramen who rushed to the scene said they saw the Reuters jeep on fire, and Shana's body lying next to it. They said that his jeep was marked "press" and that the cameraman was wearing an identifying flak jacket.
As colleagues rushed toward Shana, another missile was fired, said Wissam Nassar, a photographer with the Maan news agency. "There was an airstrike. We were thrown back, myself and another person."
Dozens of Palestinian journalists converged on the hospital where Shana was pronounced dead. Shocked, many still carrying their cameras, they wept and leaned on each other for support.
The Palestinian Journalists Union called a one-day strike for Thursday to protest the killing.
Despite near daily Israeli-Palestinian violence, casualties among journalists are rare. Only three others have been killed covering the West Bank and Gaza Strip since 1992, according to the New York-based Committee to Protect Journalists.
Reuters said Shana, 23, was killed by an explosion while covering the Israeli-Palestinian fighting. Reuters Editor-in-Chief David Schlesinger called for an investigation.
The Israeli military did not confirm its forces hit the journalist.
In separate Gaza clashes, five other Palestinian militants were killed, Palestinian officials said. The Israeli military said late Wednesday that its forces had withdrawn from Gaza.
Palestinian President Mahmoud Abbas condemned the "Israeli aggression in Gaza" and urged all sides to "cooperate with Egyptian efforts to reach a truce to halt the bloody cycle of violence." Abbas is visiting Moscow and has talks scheduled with President Bush in Washington next week.
Hamas spokesman Sami Abu Zuhri said the violence cast doubt on Egyptian cease-fire efforts. "There can be no discussion of a truce in the midst of these crimes," he said, threatening revenge against Israel.
On Wednesday morning, Palestinian militants ambushed an Israeli ground force in northern Gaza, killing three soldiers, the military said. The soldiers entered Gaza in pursuit of two Hamas militants who planted a bomb near the border and were ambushed by another Hamas force, Israeli defense officials said.
Other troops went in to the area and came under mortar fire from militants. The army said it responded with an airstrike and hit militants in the Bureij area.
Israeli government spokesman Mark Regev called the deadly Hamas ambush a "provocation," describing Israel's military operations as "defensive."
"The only logic here is that Hamas wants to sacrifice the civilian population of Gaza in order to advance its extremist and hateful agenda," he said.
The ambush was near the Nahal Oz terminal used by Israel to pump fuel into Gaza. The fuel supply was cut off last week after two Israeli civilians were killed in a Palestinian attack on the terminal - the only source of fuel for Gaza.
Israeli officials initially said the fuel deliveries would be suspended further because of Wednesday's ambush. But just hours after the attack, Israel resumed some shipments to Gaza's 1.4 million residents. It was not clear why the decision was reversed.
Mahmoud Khuzandar, deputy director of the Gaza fuel station owners' association, said eight truckloads of fuel were delivered. He said half was cooking gas and the rest was diesel fuel for Gaza's only power plant.
The fuel deliveries were expected to provide minor relief to the Gaza Strip, though they were only a tiny fraction of what the impoverished territory needs, Khuzandar said.
Israel has been cutting back on fuel and other basic supplies allowed into Gaza for months, trying to pressure the Hamas government to stop rocket attacks.
Hamas, considered a terrorist group by Israel, the U.S. and European Union, is committed to Israel's destruction.
Hamas seized control of Gaza last June from forces loyal to Abbas. Its control of the territory, along with near-daily fighting with Israel, has jeopardized Mideast peace efforts led by the United States.
Israel hopes to reach a peace agreement with Abbas' West Bank government by year's end, as both sides promised Bush last November. But Israel says it will not carry out any accord until Abbas regains control of Gaza.