Israeli airstrikes kill 15 Gaza militants (AP) GAZA CITY, Gaza Strip - Israel pounded Gaza for the second day in a row Saturday, trading airstrikes and rocket fire with Palestinian militants and killing 15 of them as the deadliest Gaza violence in over a year showed no signs of abating. Despite Egyptian efforts to mediate a cease-fire, Palestinians fired more than 100 rockets, some striking major cities in southern Israel and seriously wounding an Israeli civilian. The military responded with more than a dozen airstrikes and the targeted killings of Palestinian militants from various Gaza organizations. Israel's lauded Iron Dome missile defense system intercepted more than 25 projectiles. Still, residents were told to stay close to home and the cities of Beersheba, Ashdod and Ashkelon called off school for Sunday. Tit-for-tat exchanges between Israel and Palestinians have been routine since the 2009 war, but a flare-up of this intensity is rare. The Arab League called the Israeli attacks a "massacre." The United Nations condemned the violence and called on both sides to exercise restraint "This round in Gaza is far from being over," Israeli Defense Minister Ehud Barak said in a visit to southern Israel. We will not allow anyone to harm the citizens of the country and we will act against anyone who attempts to launch rockets. They will pay a heavy price, and no one will have immunity. The latest spate of violence got under way Friday afternoon, when an Israeli airstrike on a car in Gaza City killed top militant commander Zuhair al-Qaissi and two of his underlings. It was the highest-profile killing Israel has carried out in many months, interrupting a period of relative calm on the volatile southern front. Almost immediately, Gaza militants unleashed a barrage of rockets toward southern Israeli border communities. So far, militants have fired more than 100 rockets since al-Qaissi's killing, a major escalation from recent months. Palestinian militants fired some 50 rockets toward Israel in the previous three months. Gaza's militant Islamic Hamas rulers condemned the Israeli strike but, pointedly, their fighters did not fire rockets at Israel. Instead, they quietly allowed other smaller Palestinian militants to unleash salvos. In previous flare-ups, Hamas has used such a strategy to allow Palestinian militants to burn off their anger, with an eye toward the exchange of strikes eventually quieting down. Hamas hasn't been eager to participate in rocket barrages since Israel conducted a punishing three-week war against the militant group in 2009. Hundreds of Palestinian civilians and militants were killed and the air and ground assault destroyed much of Hamas' infrastructure. Since then, Hamas has sought to shore up its Gaza rule and amass a better weapons arsenal. Still, Israel's military said Hamas, as the territory's ruler, would "bear the consequences" for any attacks that emerged from Gaza. "We deplore the fact that civilians are once again paying the price," said Richard Miron, a spokesman for Robert Serry, the U.N.'s special coordinator for the Middle East peace process. This goes to prove finally that the situation in Gaza in very fragile and unsustainable. Egypt, which has helped arrange truces in the past, said Saturday it was trying to cobble together a cease-fire. " (We) won't give this occupation a free truce while our leaders and heroes are being killed," said Abu Mujahid, spokesman for al-Qaissi's group. Israel released a number of video clips showing militants who were apparently hit by airstrikes moments before they were to fire rockets. Palestinian rockets against Israeli communities have killed more than a dozen Israelis in the past decade. This weekend's events are the deadliest in Gaza in more than a year. Last April, Israeli killed 11 Palestinians, including four civilians, after Palestinian militants fired a rocket that hit a school bus and badly wounded a 16-year-old boy. In August, Israel assassinated Kamal al-Nairab, al-Qaissi's predecessor as leader of the Popular Resistance Committees, after the group carried out an attack from Sinai that killed eight Israelis and injured 40. Barak said al-Qaissi was preparing a similar attack. He said he couldn't say yet whether the plan had been completely thwarted. The Popular Resistance Committees is a group closely aligned with Hamas that is best known for the 2006 abduction of Israeli soldier Gilad Schalit. Schalit was freed last year in exchange for over 1,000 Palestinian prisoners. The current fighting could spiral out of control if Palestinian militants manage to kill Israeli civilians or if the Israeli strikes kill Palestinian civilians or another top militant. On Saturday, the low whooshing noise of rocket fire from border areas toward Israel was palpably heard inside Gaza City. Israeli drones hovered in the skies above. Tens of thousands of Palestinian mourners marched through the streets in funeral processions. They carried slain militants in coffins, their bodies too torn up to be wrapped in cloth, as Muslim tradition dictates. Masked militants sprayed machine gun fire above the mourners' heads in angry grief. "Revenge, revenge!" chanted the crowds. Airstrikes continued throughout the day. The latest airstrike hit militants in the southern city of Rafah, near where an elaborate network of smuggling tunnels runs between the coastal strip and Egypt's Sinai peninsula. Palestinian officials said one militant was killed and three others wounded. An earlier strike killed two Palestinian militants on a motorbike in the border town of Bani Suheila in southeast Gaza. Palestinian officials acknowledged that several of the dead were preparing to fire rockets.