Gaza erupted in the worst day of violence in a month, with at least 20 Palestinians and three Israeli soldiers dead, and putting Egyptian efforts to mediate a cease-fire in jeopardy.

Among the dead was a Reuters news agency cameraman killed while covering the conflict on Wednesday, one of several civilians among the dead — including five children, according to Palestinian officials.

Wednesday's death toll was the highest since a broad 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 minor border skirmishes.

That changed dramatically and suddenly on Wednesday, with no apparent trigger — indicating that the relative calm was more coincidence than plan.

In the day's deadliest incident, 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.

Fadel Shana, the Reuters TV cameraman, was also killed along with two bystanders, apparently in an attack in the same area, as he was filming Israeli tank movements.

Shana's last footage recorded a tank shell flying in his direction followed by a large explosion. Subsequent footage shows the Reuters jeep on fire, and Shana's body lying next to it. Shana's jeep was marked "press" and witnesses said 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 declared a one-day strike for Thursday to protest the killing of Shana. The Foreign Press Association, representing journalists in Israel and the Palestinian territories, expressed "profound sadness" and added, "His death is a stark reminder of the risks our Palestinian colleagues take every day to cover the news in Gaza."

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 into the incident.

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 U.S. President George W. 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. Egypt's efforts have been complicated by the fact that Hamas favors destruction of Israel, Israel considers Hamas a terror group and the two do not talk to each other.

U.N. Secretary-General Ban Ki-moon expressed concern Wednesday over the escalation of violence in Gaza and southern Israel and condemned the reported civilian casualties among Palestinians, including children, during Israeli military operations," said Michele Montas, a U.N. spokeswoman in New York.

She said the secretary-general "calls on Israel to abide by its obligations under international humanitarian law and human rights law" and reiterates his condemnation of rocket fire against Israeli civilian targets. He urged restraint on all sides.

A Hamas delegation arrived in Cairo for planned talks with former U.S. President Jimmy Carter on Thursday. Hamas officials said the meeting would add legitimacy to their group. Carter's meetings with Hamas officials has drawn stiff criticism from the U.S. and Israel, but Carter insists it is preferable to talk to all sides of the conflict.

The latest violence started on Tuesday night with an army operation in northern Gaza aimed at keeping suspected militants away from the border fence, the Israeli military said. During the ensuing clashes, the military said in a statement, Palestinian gunmen opened fire at Israeli soldiers from within a mosque used for storing explosives. One soldier and several Palestinian militants were wounded.

As that operation wound up in the 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 identified hitting militants in the Bureij area.

Israeli government spokesman Mark Regev called the deadly Hamas ambush a "provocation," describing Israel's military operations as "defensive." He said, "The only logic here is that Hamas wants to sacrifice the civilian population of Gaza in order to advance its extremist and hateful agenda."

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 immediately clear why the decision was reversed.

In other violence, Palestinian security officials said Israeli troops killed two Palestinian militants in a raid in the West Bank town of Qabatiya. Troops surrounded the hideout early Thursday, and exchanged fire for about an hour with the militants inside.

Islamic Jihad says one of the gunmen killed was Bilal Komel, 25, a military commander long-wanted by Israel. The second was identified as 19-year-old Ayed Zakarna.

Abu Ahmad, an Islamic Jihad, spokesman vowed a shift retribution.

"You will not escape the coming revenge," he said. "The Palestinian resistance which has humiliated the coward Zionist army will respond not by words and threats but with more blood and fire."