Israeli leaders authorised the military Tuesday to call up 40,000 reservists for a possible assault on the Gaza Strip, the army said.
"Following a directive of the chief of staff the general staff... and the approval of the government, the Israel Defence Forces increased the draft of reserve forces," the military tweeted in English.
"The approval is currently limited to 40,000."
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The move came during the most serious flare-up over Gaza since November 2012, as Palestinian militants rained rockets onto southern Israel and Israel hit back with deadly air strikes.
"We have been instructed by the political echelon to hit Hamas hard," chief military spokesman General Moti Almoz told army radio earlier, saying the operation would take place "in stages".
Defence Minister Moshe Yaalon warned it was likely to be a protracted campaign.
"We are preparing for a campaign against Hamas, which will not end in just a few days," he said in a statement after meeting security chiefs, which defined the aim as being "to exact a very heavy price from Hamas".
"We will not tolerate rocket fire on Israel cities, and we are preparing to expand the operation with everything at our disposal to strike Hamas," he said.