CAIRO/JERUSALEM, March 18 (Reuters) - Israeli airstrikes pounded Gaza, killing 326 people, Palestinian health authorities said on Tuesday, collapsing a two-month ceasefire with Hamas as Israel vowed to use force to free its remaining hostages in the strip. Israel's prime minister, Benjamin Netanyahu, said he had instructed the military to take "strong action" against the Palestinian militant group Hamas in Gaza in response to the group's refusal to release hostages held there and rejection of ceasefire proposals. Hamas accused Israel of overturning the hard-fought ceasefire deal agreed in January, leaving the fate of 59 hostages still held in Gaza uncertain.