Eight Palestinians, including four civilians and four militants, were killed Tuesday in two separate Israeli military strikes in the Gaza Strip, Palestinian spokesmen said. Israeli officials said the strikes were a response to the most serious escalation in rocket and mortar fire from the coastal territory since the 2009 Israeli offensive that sought to end such attacks.

The Palestinian dead included a 50-year-old man and three youths ages 11, 16 and 17 who were playing soccer in the eastern part of the Gaza Strip, said Adham Abu Silmiya, a Palestinian medic and spokesman for the Palestinian ambulance service in Gaza. Twelve others were injured, four critically, he said.

Lt. Col. Avital Leibovich, a spokeswoman for the Israel Defense Forces (IDF), said the army was aware of the Palestinian reports of dead children and was investigating the incident.

Prime Minister Binyamin Netanyahu later issued a statement expressing regret that innocent civilians had been unintentionally hit by IDF shelling, but said that the military was responding to Hamas fire targeting Israeli communities. He accused Gaza's militant ruling group Hamas of using Palestinian civilians as human shields.

"While the State of Israel has no intention of bringing about a deterioration of the situation," he added, "the IDF will continue to take determined action to defend Israeli citizens."

Later Tuesday, the Palestinian militant group Islamic Jihad said a separate Israeli airstrike in Gaza had killed four of its members. The Israeli air force "identified a group of terrorists in the northern Gaza Strip who were preparing to fire military-use projectiles at the Israeli home front, and thwarted the attempt by firing at them, confirming a hit,'' the military said.

The military said the four killed were known to have fired a rocket at the southern city of Beersheba last month.

Although Abu Silmiya said the four people killed in eastern Gaza were the victims of Israeli tank fire, the Israeli army said it had fired mortars in response to four military projectiles launched from the northern part of Gaza that landed in the area of the Sha'ar Hanegev regional council inside Israel.

Tahar al-Nounou, a spokesman for Hamas's government in Gaza, said that no one had fired rockets from the area targeted by Israel on Tuesday. "The Palestinian government condemns strongly the awful crime that was committed by the Zionist occupation this afternoon,'' he said in Gaza.

Palestinian groups inside the Gaza Strip, including one calling itself the al-Nasr Brigade and another the Mujahideen Brigades, acknowledged they had fired rockets into Israel on Tuesday. And earlier in the day, the Israeli army said it had thwarted an attempt by Palestinians to launch an antitank missile.

According to the Israeli military, 130 mortar shells and rockets fired from the Gaza Strip have landed in Israel since the beginning of the year, including 56 in the past week.

Although Hamas has at times worked to prevent attacks into Israel, three days ago the group asserted responsibility for firing 50 mortar rounds, prompting an Israeli complaint to the United Nations and raising fears of an otherwise low-level confrontation becoming a broader conflict.

Israel unilaterally withdrew its troops and settlers from the coastal strip in 2005 but still limits movement of people and goods in and out of the territory.

The increase in attacks comes as Hamas - which has controlled the Gaza Strip since battling with its rival Fatah for control of the territory in 2007 - is again in tenuous discussions with Palestinian President Mahmoud Abbas about possible reconciliation between the factions.

The heightened violence follows calls by Palestinians in the West Bank to end the political divisions with Hamas. Some observers in Gaza have speculated that the increased Palestinian mortar and rocket fire could be an attempt to create a distraction and to undermine any reconciliation effort.