Gaza conflict: Egyptian PM visits Gaza as rockets continue to fall Even though about 50 rockets have fallen in Israel over the past two hours, we chose not to attack in Gaza due to the visit of the Egyptian prime minister. Hamas is lying and reporting otherwise," the army said in a Twitter message. Meanwhile, air raid sirens sounded in Tel Aviv and an explosion was heard, witnesses said. Hamas militants in the Gaza claimed they had fired a rocket at Israel's commercial centre. A police spokesman acknowledged an explosion had been heard but said there was no initial indication a rocket had struck the city nor any immediate reports of casualties or damage. Palestinian militants in Gaza launched two rockets toward the city on Thursday. One fell in the sea, a security source said, and the other landed in a Tel Aviv suburb, causing no damage or casualties. It was the first time Israel's biggest city had been alerted by air raid sirens since the 1991 Gulf War. Hassan Nasrallah, the head of the Lebanese group Hezbollah, said the targetting of Israel's commercial centre was a "very significant development" in the conflict with the Jewish state. "The firing of Fajr 5 rockets on Tel Aviv today shows the maturation, the wisdom and strength, and the courage of the Palestinian resistance in the Gaza Strip," Nasrallah said in a speech on Thursday night. "The Israeli enemy was surprised and forced to acknowledge that its capital had been hit," said the leader of Lebanon's most powerful military force, adding that the Iranian-made rockets have a range of 44 miles. The threat to Tel Aviv contributed to Israel's decision to mobilise troops on its border with Gaza. At least a dozen trucks carrying tanks and armoured vehicles were seen moving toward the border area, while buses ferried soldiers, as Israeli forces moved closer to a ground war against the radical Islamist group Hamas. One hit the town of Rishon Lezion, seven miles away, while a second, an Iranian Fajr-5 missile, fell into the sea off the coast of Jaffa, just to the south of the city. Air raid sirens sounded in the city for the first time in 20 years and sent residents running for shelters. Israeli officials had earlier indicated a strike against Tel Aviv would be a "red line" which could trigger a ground war. "This escalation will exact a price that the other side will have to pay," Mr Barak warned last night in the wake of the rocket attacks. He also spoke to an "extremely concerned" David Cameron by telephone on Thursday night. Last night, Downing Street declared Hamas bears "principal responsibility" for the escalating crisis, branding rocket attacks on Israel "completely unacceptable." So far, 19 Palestinians, including both militants and civilian men, women and children, have been killed by Israeli air strikes, while three Israeli civilians died when a rocket fired from Gaza hit their apartment block in the southern Israeli town of Kiryat Malachi yesterday morning.