Gaza's worst day of violence in a month, in which at least 20 Palestinians and three Israeli soldiers died, appeared to jeopardize Egypt's efforts to mediate a Middle East cease-fire.

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 Gaza's Hamas rulers appeared to be honoring an informal truce, though punctuated with Palestinian rocket attacks, some Israeli airstrikes and minor border skirmishes.

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

Hamas spokesman Sami Abu Zuhri said the violence cast doubt on Egyptian cease-fire efforts. A Hamas delegation arrived in Cairo for planned talks with former President Jimmy Carter on Thursday.

"There can be no discussion of a truce in the midst of these crimes," Zuhri said, threatening revenge against Israel.

Egypt's efforts already are 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.

Hamas officials said their meeting with Carter would add legitimacy to their group. Carter has drawn stiff criticism from the U.S. and Israel for meeting with Hamas, but Carter insists it is preferable to talk to all sides of the conflict.

Palestinian President Mahmoud Abbas, a Hamas rival whose moderate government runs the West Bank, 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."

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 ages 12 to 15, were killed, said Dr. Moaiya Hassanain of the Palestinian Health Ministry.

Reuters news agency cameraman Fadel Shana was killed along with two bystanders, apparently in an airstrike in the same area, as he was filming Israeli tank movement.

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.

The Palestinian Journalists Union declared a one-day strike for Thursday in protest. The Foreign Press Association 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."

The Israeli military did not confirm its forces hit the 23-year-old journalist. Reuters Editor-in-Chief David Schlesinger called for an investigation.

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 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 came under mortar fire. 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."

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.

Hours after the attack, Israel resumed some shipments to Gaza's 1.4 million residents.

The Israeli military said late Wednesday that its forces had withdrawn from Gaza.

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."