(AP) -- Cross-border fighting between Gaza and Israel, touched off by Israel's killing of a top militant leader, showed no signs of letting up on its fourth day.
Gaza militants fired dozens of rockets at Israeli towns, hitting an empty school Sunday, and Israeli airstrikes killed three Gazans, including a boy and a farm guard.
Egypt tried to mediate but failed to end the worst violence in more than a year that has killed 18 Gazans, all but two of them militants, and disrupted the lives of some 1 million Israelis living in Gaza rocket range.
Even so, Israel and Gaza's Hamas rulers seemed eager to avoid a full-scale conflict. A three-week war in three years ago left both sides badly bruised, Israel in the diplomatic arena and Hamas on the battlefield.
In the current round, Hamas has pointedly kept its large rocket arsenal and thousands of fighters out of the confrontation, even though it has not tried to stop two smaller Gaza groups, Islamic Jihad and the Popular Resistance Committees, from launching rockets and mortars.
Israeli Defense Minister Ehud Barak acknowledged that Hamas did not take part in the rocket salvos. Up to now Israel has blamed Hamas for all violence from Gaza because it rules the territory.
Israel's military chief, Lt. Gen. Benny Gantz, said Sunday that "we are not interested in escalation in and of itself."
On a visit to southern Israel, Prime Minister Benjamin Netanyahu pledged the airstrikes would continue as long as necessary. "We have a clear policy: we will hit anyone who plans to harm us, who prepares to harm us and who harms us," he said in a meeting with local leaders.
Israel said it launched Friday's initial airstrike, which killed PRC leader Zuhair al-Qaissi in a car in downtown Gaza, to stop a plan by his splinter group to infiltrate into Israel through Egypt's lawless Sinai Peninsula. Israel says the PRC was behind an attack on the border in August, killing eight Israelis.
Palestinians across the political spectrum accused Israel of deliberately escalating tensions. The groups involved in firing rockets dismissed truce offers presented by Egypt.
"We will not give calm for free, and the blood of our leaders and martyrs will not be spilled in vain," said a PRC spokesman who uses the nom de guerre Abu Mujahed.
Egypt was trying to broker a truce but insisted Israel stop its airstrikes first, said Yasser Othman, Egypt's envoy to the Palestinian Authority. Representatives of militant groups were in Cairo for talks.
A top Islamic Jihad leader, Ziad Nakhala, denied reports that his group had agreed to stop firing rockets. Rocket fire toward Israel continued for a fourth day early Monday, keeping schools closed in the southern part of the country for the second day in a row.
In fresh Israeli airstrikes, a group of rocket launchers were targeted in the northern Gaza Strip.
Another strike in Gaza City wounded some two dozen people, including several children, health officials said.
In the past, similar flare-ups have died out by themselves or with informal cease-fires negotiated by third parties, but there is always a danger of sudden escalation if an attack by either side causes multiple casualties.
An uneasy informal truce has held on the Gaza-Israel border since the Gaza war that started at the end of 2008, though smaller Gaza groups fire rockets and mortars from time to time with Hamas looking the other way.
Hamas itself has largely observed the truce, in part because it has made safeguarding its five-year-old rule in Gaza a priority and does not want to provoke harsh Israeli retaliation.
Trying to claim a middle ground, Hamas leaders issued fiery statements Sunday in support of those firing rockets, but Gaza Prime Minister Ismail Haniyeh also noted that "the brothers in Egypt are working around the clock to stop the (Israeli) aggression."
The U.S. and the U.N. called on both sides to show restraint.
Israeli airstrikes Sunday killed a 12-year-old boy, a 60-year-old farm guard and a militant, said Gaza health official Adham Abu Salmia. The boy was hit while walking with a friend to school in the northern town of Jebaliya and the guard died in Gaza City while walking with his dog, who was also killed, he said. The militant was killed at a rocket launching site, he said.
Hundreds of mourners attended funerals in two Gaza towns Sunday. Women wailed and men chanted slogans demanding revenge for the deaths, as militants fired guns into the air. The boy's body was draped in the black flag of the Islamic Jihad militant group.
On Sunday, Palestinians fired more than 30 rockets at southern Israel. One struck the courtyard of an empty school in Beersheba, police said. Sunday is a school day in Israel, but schools in rocket range were closed to try to prevent casualties.
Israel claimed success in the first major battlefield test of its of its Iron Dome anti-missile system, which intercepted 30 of more than 120 rockets fired from Gaza since Friday, Barak told Israel's weekly Cabinet meeting, according to a statement from his office. He said anti-rocket batteries were deployed near three southern cities close to Gaza.