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 were killed, and Palestinians reported 19 deaths, including at least three killed late Thursday. Hamas gave conflicting information as to how many of them were Hamas militants.

At least 300 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 130, the Israel Defense Forces said. The al-Qassam Brigade, Hamas' military arm, said on its Twitter feed that it had shot 527 projectiles at Israel in that time.

Q&A: Gaza strikes could be beginning of ground attack

One rocket struck an open area near Rishon LeZion, an Israeli city with more than 200,000 residents just south of Tel Aviv, the IDF said.

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.

Israeli Prime Minister Benjamin Netanyahu was in the building at the time of the evacuation, ministry officials said.

Israel has targeted more than 300 "terror sites" in Gaza, the IDF said. The military said it targeted scores of "medium and long range rocket launch and infrastructure sites across the Gaza Strip." The Israeli navy has taken aim at targets along Gaza's shoreline, the IDF said.

Sources with Hamas, which controls the government in Gaza, and Palestinian Islamic Jihad said that more than 140 strikes had hit Gaza.

The al-Qassam Brigade said its operatives downed a military drone east of Gaza. An Israeli military spokeswoman told CNN that no IDF aircraft was shot down.

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.

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 the child appears to be spattered with blood.

The al-Qassam Brigade tweeted a screen shot from Hamas-run al Aqsa TV, showing the mangled body of a child. "Israel's military kills Palestinian children in cold blood in #Gaza," the tweet said.

Al-Aqsa TV quoted the health ministry as saying 19 people had been killed, among them six children and two "elderly."

The channel said more than 180 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.

Israel denies attacks linked to upcoming election

Hamdan insisted that Israel "started the war."

"We are defending ourselves," he said, arguing that 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.

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 moved 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 operations in the past. The U.S. government and the European Union consider Hamas a terrorist organization.