A fireball erupts from an Israeli airstrike in Gaza City on Monday. AFP

Palestinian 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.

Fireballs repeatedly lit up Gaza City on Monday night as explosions sounded and sirens wailed.

Palestinian group Hamas, which dragged off about 150 people in its surprise weekend assault on Israel.

Hamas — which the Israeli army estimates sent about 1,000 fighters across the border, spraying gunfire at civilians — said on Monday that Israeli air strikes had killed four of the hostages.

It later said it could start killing them itself.

"Every targeting of our people without warning will be met with the execution of one of the civilian hostages," the Ezzedine Al Qassam Brigades, the armed wing of Hamas, said in a statement.

Agence France-Presse