Israeli troops and tanks moved toward the border of the Gaza Strip today and the government authorized the call-up of reservists as air strikes failed to halt the rounds of rockets being fired from the Hamas-controlled territory.
The U.S. State Department today blamed Hamas for the escalation of violence.
Click here for scenes from Gaza during the air strikes and rocket attacks.
"This is violence instigated by Hamas," said Mark Toner, the department's deputy spokesman. "As we've said very clearly in our statement yesterday, that we extend our sympathies to the victims, innocent Israeli and Palestinian civilians that have been affected by this violence. But let's be very clear where the onus lies."
Israeli air strikes that hit more than 150 targets across Gaza since the operation dubbed "Pillar of Defense" began Wednesday evening.
The first strike was on Ahmed Jabari, the chief of staff of the military wing of Hamas, the Ezzedeen al-Qassam Brigades.
It was followed by a wave of airstrikes on other militants, buildings and installations, notably launching sites and rockets, which included the long-range Fajar rockets that could reach Tel Aviv.
Hamas sources said a dozen Palestinians have been killed, including at least two children. One child was just 11 months old.
"What did my son do to die like this?" the child's father cried.
Today, Gaza's Hamas Prime Minister Ismail Haniyeh vowed revenge for Jabari's death.
"His blood will not be in vain," Haniyeh said.
Hamas and other militant groups retaliated against Israel with a fresh barrage of rockets that killed at least three Israelis -- the first Israeli deaths in the escalation -- in the southern town of Kiryat Malakhi.
At least one rocked from Gaza militants landed in Tel Aviv, Israel's commercial capital, as sirens blared and residents ran for cover.
Hamas, Islamic Jihad and other militant groups have fired more than 200 rockets into southern Israel today, according to Israel's military.
"We are still at the beginning of the event, not at the end, and we expect some complicated tests ahead," Israeli Defense Minister Ehud Barak said in a nationally televised address Monday night, standing next to Prime Minister Benjamin Netanyahu.
According to The Associated Press, Israel has started moving its troops toward Gaza and has called up reservists in anticipation of a ground operation. Military officials, however, had not decided whether to enter Gaza, the AP reported.
Israeli officials say the operation is in response to more than 800 rockets that have landed in Israel this year.
Barak laid out the four goals of "Pillar of Defense": to strengthen deterrence, damage the rocket launching network, hit Hamas and its allies and to minimize the damage in Israel.
When the Israeli offensive began Monday, its military tweeted, "We recommend that no Hamas operatives, whether low level or senior leaders, show their faces above ground in the days ahead."
Hamas fired back saying that Israel "opened the gates of hell" by assassinating Jabari.