GAZA CITY, Gaza Strip — An airstrike killed four Palestinians as they were firing mortars at Israel on Sunday, the latest in a surge of clashes that have rocked a 5-month-old truce between Israel and Gaza's militant Hamas rulers.
In a separate incident, Palestinian rocket fire injured one Israeli.
Both sides say they would like to preserve the truce, which expires next month. But with violence escalating over the past two weeks, the cease-fire appears to be unraveling.
The militants killed in the airstrike were from a small Hamas-allied group known as the Popular Resistance Committees. Abu Attaya, a spokesman for the group, said the four were firing mortars when they were killed.
Abu Mujahed, another spokesman for the Popular Resistance Committees, said his group is pulling out of the truce, and Hamas spokesman Fawzi Barhoum threatened retaliation for the killings.
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"Today's crime will not pass without punishment," he said.
After the strike, dozens of onlookers converged on the bodies of two of the dead men, shouting, "God is great!" They then carried the bodies, one wrapped in a blanket, to a nearby civilian car.
At sundown, a rocket launched from Gaza hit a house under construction in the battered Israeli border town of Sderot, slightly injuring a resident, according to police and media reports. Palestinians launched two other rockets earlier in the day, but no one was hurt, the military said.
According to the Israeli military's count, Palestinians have sent more than 170 rockets and mortars flying at Israel since the violence resumed nearly two weeks ago. Israeli troops have killed 15 militants, and two more died in unclear circumstances.
Israeli leaders signaled they have not given up on the fragile cease-fire.
Speaking ahead of the Cabinet's weekly meeting, Prime Minister Ehud Olmert said he had told the military to present plans for "various operations against Hamas' terrorist regime" in Gaza, but gave no time frame.
"We are not eager for battle, but we do not fear it."
In Gaza, the Hamas government of Prime Minister Ismail Haniyeh said continuing the calm "depends on stopping the aggression against our people, opening the border crossings and all the terms of the calm."
Barak decided late Sunday to keep the Gaza crossings closed today because of the rocket attacks.
The closures bar badly needed goods and fuel from entering the impoverished territory, home to 1.4 million people. U.N. food supplies in Gaza have been depleted, and the fuel cutoff has led to power shortages.
Olmert is set to meet today with Hamas' rival, moderate Palestinian President Mahmoud Abbas, for another in a series of talks on peace negotiations.

