GAZA CITY (CNN) -- Rockets and shells crisscrossed the skies over southern Israel and Gaza on Thursday as Palestinian militants continued rocket attacks on Israeli civilians and Israel pounded what it called terror sites.
Israel reported three people killed. Hamas said 13 Palestinians were killed, but gave conflicting information as to how many of them were Hamas militants.
At least 245 rockets from Gaza have been fired into Israel since "Operation Pillar of Defense" began Wednesday, the Israeli military said. Israel's Iron Dome defense system has intercepted more than 80, the Israel Defense Forces said.
One rocket struck an open area near Rishon LeZion, an Israeli city with more than 200,000 residents, the IDF said. It is just south of Tel Aviv.
Also, air sirens went off inside and outside the Israeli Defense Ministry in Tel Aviv. The building was evacuated briefly, and employees were allowed back in when the sirens stopped. An explosion was heard far off in the distance.
Israel has targeted more than 200 "terror sites" in Gaza, "severely damaging the capabilities of Palestinian terrorist organizations," the IDF said. The military said it targeted scores of "medium and long range rocket launch and infrastructure sites across the Gaza Strip."
Sources with Hamas, which controls the government in Gaza, and Palestinian Islamic Jihad gave similar figures, saying that more than 140 strikes have hit Gaza and that scores of rockets and shells have been fired into Israel.
At least three Israelis were killed and four were wounded when a rocket struck an apartment building in the town of Kiryat Malakhi on Thursday, an Israeli police spokesman said. "Good morning to our friends in America," the IDF tweeted. "While you were sleeping, 3 Israelis were killed when a rocket hit their house."
Israeli military spokeswoman Avital Leibovich tweeted a photo that she said was a baby wounded from a rocket attack in Israel. The baby's face is blurred, but she appears to be spattered with blood.
Thirteen people, including two children, were killed in airstrikes in Gaza on Thursday, said Dr. Asraf el-Qdra of Medical Aid for Palestinians. Of the casualties, nine were Hamas militants, a Hamas source told CNN.
But Hamas spokesman Osama Hamdan told CNN the 13 dead included five children. He identified only two as Hamas militants.
Palestinian officials said more than 120 people have been wounded since the Israeli strikes began this week in Gaza. Israel has reported several people wounded, including another three soldiers injured Thursday morning by rockets from Gaza.
Hamdan insisted that Israel "started the war."
"We are defending ourselves," he said, arguing that Israeli Prime Minister Benjamin Netanyahu was looking to cement support in advance of an election in two months.
Israeli Defense Minister Ehud Barak denied that any politics are involved in the decision.
Speaking to CNN, Barak said Israel has destroyed most of the "heavy long range rockets" used by militants in Gaza and is working to "systematically destroy" installations in which other rockets are produced.
"It will take some time," he said, "and we are not going to stop until the whole thing will dramatically change" -- with an end to the attacks from Gaza, he said.
Israeli forces are going after Hamas weapons, storage bunkers, weapons labs and workshops, an Israeli official told CNN. The official has direct knowledge of Israeli plans but declined to be identified because of the sensitive nature of the information.
The Israeli army is moving nearly a division's worth of troops -- perhaps 1,500 to 2,000 -- to the border, the official said.
While multiple militant groups are behind the rocket attacks, Israel holds Hamas responsible ever since it took control of Gaza, Barak said.
Hamas' military wing has claimed responsibility for numerous terrorist operations in the past. The U.S. government and the European Union consider Hamas a terrorist organization.
Netanyahu issued a statement Thursday saying, "In recent days and weeks, Hamas and the other terrorist organizations in Gaza have made normal life impossible for over one million Israelis. No government would tolerate a situation where nearly a fifth of its people live under a constant barrage of rockets and missile fire. ... This is why my government has instructed the Israeli Defense Forces to conduct surgical strikes against the terrorist infrastructure in Gaza."
He added, "There is no moral symmetry; there is no moral equivalence, between Israel and the terrorist organizations in Gaza. The terrorists are committing a double war crime. They fire at Israeli civilians, and they hide behind Palestinian civilians. And by contrast, Israel takes every measure to avoid civilian casualties."
Ghazi Hamad, Hamas' deputy foreign minister, told CNN that Hamas was sending rockets toward Israel's population because Israel thinks "that it is easy to kill people in Gaza," enter the area and "do