More than 200 Palestinians have been killed in an Israeli air assault on the Gaza Strip, which is ruled by the Islamic militant group Hamas.

Israeli warplanes and helicopters launched a massive assault on Hamas installations across Gaza. Military sources say 40 targets were destroyed in a span of just five minutes. Hamas was taken by surprise. There were heavy casualties when a graduation ceremony for new military officers was hit by missiles.

Israeli Foreign Minister Tzipi Livni said it was retaliation for Hamas rocket and mortar attacks, which escalated after a six-month ceasefire expired a week ago. "There is no other alternative and we are doing what we need to do in order to defend our citizens. Israel was attacked from Gaza Strip. Enough is enough," she said.

Defense Minister Ehud Barak said the aim of the operation is to deal a severe blow to Hamas. Barak said the offensive will be expanded as necessary. He said the war on terror is a long one, but Israel is determined to change the situation in the South.

Israel has not ruled out a ground offensive, and officials say the Hamas leadership could be targeted.

The internationally backed Palestinian government in the West Bank expressed outrage over the air strikes. Palestinian negotiator Saeb Erekat said "We condemn with the strongest possible terms this savage aggression against Gaza Strip."

Hamas promised revenge with rocket attacks and suicide bombings.  Let one thing be clear," said Hamas spokesman Taher a-Nunu, "Hamas will never raise the white flag." Exiled Hamas leader Khaled Meshaal, in an interview with Al-Jazeera television, called for a new Palestinian uprising, or Intifada, against Israel.