The Israeli Air Force struck some 70 targets in the Gaza Strip in one hour's time, the IDF Spokesman's Office said Thursday night. Among the targets, the IDF statement said, were underground medium-range rocket launching pads. The most recent blitz of air strikes brought the total number to some 320.Palestinians have fired over 300 rockets from Gaza into the South since the IDF launched its operation. Two rockets triggered an air raid siren in Tel Aviv, marking the first time that a real siren was sounded in Tel Aviv since the Gulf War in the early 1990s. According to Bethlehem-based Ma'an news agency, 16 Palestinians have been killed since the start of the operation. At least two children, including an 11-month-old baby were among the casualties.The IDF shifted infantry brigades and tank columns to the Gaza border, and announced the call-up of 30,000 reservists, as it made preparations for a possible ground incursion into the Gaza Strip.“The aim of the attacks is to disrupt the ability of the terrorist organizations to fire rockets,” the IDF said. “Terrorist cells involved in firing rockets have also been hit.”The targets were chosen after intelligence reports on their locations were gathered over recent months.“The IDF is landing painful blows against Hamas and against terrorist infrastructure,” Defense Minister Ehud Barak said. He said the rocket fire on greater Tel Aviv was “an escalation, and the other side will have to pay a price for this escalation.Earlier on Thursday, Barak hinted that Operation Pillar of Defense against Gaza-based terrorists could be a lengthy battle.Addressing the possibility of a ground operation, the defense minister said he had instructed the IDF to prepare for “any type of development, if and when necessary.”On Wednesday, the IAF struck and killed Hamas military chief Ahmed Jabari, marking the start of the operation. One other person was killed in the attack.Following the assassination, the IAF struck over 20 underground rocket launchers belonging to Hamas and Islamic Jihad. The airstrikes targeted long-range rockets in the possession of terror organizations, such as the Fajr-5 and other rockets that are capable of striking Tel Aviv from Gaza. Palestinian sources said that six Gazans were killed in the IDF strikes.In response to the attacks, Hamas said that "the occupation has opened the gates of hell."Senior Hamas official Izzat al-Rishq said the assassination will not "break the will of our people, nor weaken our resistance," Al Ahram reported. Al-Rishq made the comments on his Facebook site, adding that Prime Minister Binyamin Netanyahu is using the "war crime" to boost his chances of reelection, but that it could "cost him his political future."Wednesday's violence came after a four-day rocket barrage which began Saturday appeared to have come to an end on Tuesday evening. The hostilities saw over 100 rockets fired from the Gaza Strip into Israel and Israeli retaliatory strikes which killed six Palestinians.Both Israel and Hamas sent signals to each other via Egypt that they would hold their fire unless attacked, after five days of mounting violence.Yaakov Lappin and Reuters contributed to this report.