Security forces raised their alert level over the weekend after Hamas effectively ended a 16-month truce, announcing it would resume attacks following an explosion - possibly caused by IDF artillery fire - that killed seven civilians in the Gaza Strip Friday. Hamas's armed wing, Izaddin al-Kassam, said its members had fired at least 15 rockets at Sderot and other Israeli communities since Friday. The announcement was the first of its kind since Hamas and other Palestinian armed organizations agreed on a truce in Cairo in February 2005. The IDF said that between 25 and 30 Kassam rockets and mortar shells had been fired at Israel since Friday night. Friday's incident was preceded by the targeted killing of seven terrorists involved in Kassam rocket attacks, including Jamal Abu Samhadana, commander of the Popular Resistance Committees and head of Hamas's new security force in the Gaza Strip, who was killed by an IAF bomb late Thursday night at a PRC training camp. Minutes after the explosion, an IAF aircraft fired a rocket killing two Hamas members said to have been responsible for the many Kassam attacks on Sderot last week. The blast, on the Gaza beach between and Sudaniya and Beit Lahiya Friday, scattered body parts, destroyed a tent and sent bloody sheets flying into the air. A panicked crowd quickly gathered, screaming and running around hysterically. A sobbing girl lay in the sand, crying for her father. "Father! Father!" she screamed. The body of a man lay motionless in the sand nearby. Palestinian officials said seven people were killed and more than 30 wounded. Hardest hit was the Ghalia family, which lost six members, among them the father, one of his two wives, an infant boy and an 18-month-old girl. Hamas issued a statement vowing to "respond to each Israeli atrocity until the liberation of all Palestine." In a reference to suicide bombings, the statement said: "The Zionist massacres are a declaration of war. This means that the earthquake in Zionist cities will resume." Senior officers said they were gearing up for the possibility of an escalation in violence. The Shin Bet (Israel Security Agency) recorded close to 90 terror threats on Saturday alone. "There are predictions the incident [on Friday] will not go over quietly," one officer said. Hamas leader Khaled Mashaal, in a statement issued from Damascus to all Palestinian factions, called for putting their differences aside and "mobilizing their energies and forces to respond to the Zionist aggression and to reactivate the intifada." Defense Minister Amir Peretz urged Palestinians to show restraint and refrain from launching retaliatory attacks. "We are trying very hard not to escalate the conflict," Peretz told reporters during a visits Saturday to the Gaza division. But he said the IDF would continue acting against terrorists, including Hamas officials. Despite the tragic incident on Friday, Peretz vowed to keep on targeting terrorists. "We know of Hamas elements directly involved in terror," Peretz said. "We plan to take action against anyone who plans terror attacks. There is no insurance certificate for anyone and if someone endangers the lives of Israelis they will be targeted." Despite the threats, Hamas leaders in the West Bank and Gaza Strip stopped short of officially declaring the end of the truce. Although PA Prime Minister Ismail Haniyeh and his ministers used harsh rhetoric to condemn the killings, saying Palestinians maintained the right to retaliate, none of them called for ending the truce. "This is a war crime and a bloody message aimed at breaking the will of our people and forcing them to surrender," Haniyeh told reporters after visiting the wounded in hospital. "The political dispute in the Palestinian arena must end and we should join forces against the [Israeli] aggression." Haniyeh said he intended to adopt Huda Ghalia, the 12-year-old girl whose family was killed in the explosion Friday. PA officials said Hamas was not interested at in ending the truce at this stage. "They have a lot to lose if they return to the attacks on Israel," said one official. "They are now in government and they don't want to give Israel an excuse to attack them. They also don't want the international community to treat then as terrorists." Another official said that Mashaal and other Hamas leaders living abroad - who often express more radical views than their colleagues in the West Bank and the Gaza Strip - had also refrained from declaring an end to the truce. "They are all calling on their supporters to respond to the Israeli attack and that's all," the official said. "I believe that after a day or two they will stop the rocket attacks. It all depends on whether Israel escalates the situation by targeting Hamas members and leaders." Hamas also refrained from declaring an end to the truce following the killing on Thursday night of Samhadana, 43, who was wanted by Israel for a series of attacks over the past decade. PA President Mahmoud Abbas, accusing Israel of waging a "war of genocide," declared three days of mourning in the West Bank and Gaza Strip. "Undoubtedly, what happened in the Gaza Strip was a bloody massacre against civilians," he said. "Israel is perpetrating one of the biggest crimes of human genocide against the Palestinians." The armed wing of Abbas's Fatah party, the Aksa Martyrs Brigades, said in a statement that it would resume suicide attacks in response the killing of the civilians. The group also threatened to kidnap soldiers and to launch attacks on settlers and soldiers in the West Bank. AP contributed to this report.