NEW: "Israel has no intention of bringing about a deterioration of the situation," Netanyahu says. NEW: "The massacre... is a war crime," says Hamas. Four Palestinian militants were killed Tuesday evening. Israel says mortars and rockets have been fired from Gaza into southern Israel.

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Four Palestinian militants were killed Tuesday evening in an Israeli airstrike in the Zeitoun neighborhood east of Gaza City, the latest in a series of attacks in recent days that have included two children among the fatalities, Hamas security sources and Palestinian medical officials said. Islamic Jihad said all four militants were field commanders for Islamic Jihad. An Israeli army spokeswoman said that an Israeli military aircraft fired toward the group after identifying a group of militants who were preparing to launch a rocket into Israel. In an earlier incident Tuesday, two adults and two children were killed in the Al-Shajaieh neighborhood east of Gaza City when shells from Israeli tanks hit a house there, the sources said. The children, 11-year-old Mohammed Jihad Al-Hilo and 16-year-old Yasser Ahed Al-Hilo, were playing soccer outside their house when the shells hit, the sources said. Yasser Hamed Al-Hilo, 50, and Mohamad Saber Harara, 20, were also killed, according to the sources. Eight others were wounded, three of them critically, they said. The Israel Defense Forces said the mortar attack was carried out in response to the launch from Gaza of four military-use projectiles into the Shaar Hanegev regional council area. "It appears that uninvolved civilians were injured as a result," the IDF said. The incident was under investigation by the IDF. "The IDF does not want an escalation of violence in the South," an IDF spokesman said. In a statement, Israeli Prime Minister Benjamin Netanyahu expressed regret over the deaths and injuries of Gaza civilians in Tuesday's Israeli military action and said the strikes were carried out in response to Hamas attacks on Israeli citizens. "It's unfortunate that Hamas continues to intentionally rain down dozens of rockets on Israeli civilians using its own civilians as shields," he said. "Israel has no intention of bringing about a deterioration of the situation, but at the same time the IDF will continue to act decisively to protect Israeli citizens." Hamas condemned the killings east of Gaza City. "The massacre in Al-Shajaieh neighborhood is a war crime and the Israeli occupation bears full responsibility," it said in a statement. Palestinian Prime Minister Salam Fayyad also condemned what he called "the Israeli aggression against the civilians" and asked for international protection of civilians. He called the military shelling of residential neighborhoods a "dangerous escalation that should be ended." Since Saturday, 10 Palestinians have been killed and at least 37 others wounded in a series of incidents in Gaza, Palestinian medical sources said. Also since Saturday, 61 mortars and rockets have been fired into southern Israel, the IDF said. CNN's Michal Zippori, Kareem Khadder and Talal Abu Rahma contributed to this report.