The escalation in the conflict was sparked by weeks of tensions in contested Jerusalem.
Since sundown on Monday when the cross-border attacks began, 24 Palestinians – including nine children – were killed in Gaza, most by air strikes, Gaza health officials said.
The Israeli military said 15 of the dead were militants.
During the same period, Gaza militants fired more than 250 rockets towards Israel, injuring six Israeli civilians in a direct hit on an apartment building.
In a further sign of rising tensions, Israel signalled it is widening its military campaign.
The military said it is sending troop reinforcements to the Gaza border and the defence minister ordered the mobilisation of 5,000 reserve soldiers.