GAZA CITY, Gaza Strip (AP) — Palestinian militants barraged Israel with more than 200 rockets on Thursday, killing three people as Israel pressed a punishing campaign of airstrikes on militant targets across the Gaza Strip. Three rockets targeted the densely populated Tel Aviv area, setting off air raid sirens in brazen attacks that threatened to trigger an Israeli ground invasion of Gaza. Late in the day, Israel signaled a ground operation may be imminent as forces moved toward the border area with Gaza. At least 12 trucks were seen transporting tanks and armored personnel carriers, and a number of buses carrying soldiers arrived. Israeli Defense Minister Ehud Barak said he had authorized the army to call-up additional reservists for possible action. The army said it was prepared to draft up to 30,000 additional troops. "I ordered the military today to widen the draft of reserve soldiers in order to be able to be ready for any development," Barak said. Military officials said the moves were to prepare for the possibility of a ground invasion, but stressed no decision had been made. Israel TV stations, however, said a ground offensive was expected Friday. The fighting, the heaviest in four years, has also killed 15 Palestinians in two days and brought life to a standstill on both sides of the border. School has been canceled and many were huddling indoors. Israel and Hamas have largely observed an informal truce for the past four years. But in recent weeks, the calm unraveled in a bout of rocket attacks on Israel from Gaza and retaliatory Israeli airstrikes.