The biggest wave of strikes since the ceasefire began in January comes after the Israeli military said talks to extend the ceasefire failed.
Hamas accuses Israel of attacking "defenceless civilians", and says mediators should hold Israel "fully responsible" for "violating and overturning" the ceasefire.
Prime Minister Benjamin Netanyahu's office says Israel will "from now on act against Hamas with increasing military strength", accusing Hamas of "repeated refusal to release our hostages", and of rejecting proposals to extend the ceasefire.
In a statement, Hamas says the United States "bears full responsibility for the massacres" in Gaza, after it emerged Israel briefed the US on the strikes prior to launching them.
