Palestinian militant group Hamas, which dragged off about 150 people in its surprise weekend assault on Israel, threatened to execute the hostages if Israeli air strikes continue "targeting" Gaza residents without warning.
The threat came after Israel on Monday imposed a total siege on the Gaza Strip, cutting off food, water and electricity supplies, and sparking fears of an increasingly dire humanitarian situation.
Israel has been left reeling from Hamas's unprecedented ground, air and sea attacks, likening them to 9/11.
The death toll rose to 900 in Israel, which has retaliated with a withering barrage of strikes on Gaza, raising the death toll there to 687.