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The announcement was made on the official website of the Israeli army, and follows a week-long bombardment of Gaza that has left at least 2,200 dead and thousands wounded.
It came as Israeli Prime Minister Benjamin Netanyahu visited frontline troops.
The expected offensive – a long-feared full-scale invasion of Gaza – follows the October 7 incursion by Hamas militants into Israel that left about 1,300 people dead. The majority of those killed on both sides are civilians. The Israeli army has built up a force of about 300,000 soldiers for the attack.
The Israeli military said their forces, “backed by an extensive logistical effort and by completing the reserve mobilisation of hundreds of thousands of servicemen, are preparing to implement a wide range of offensive operative plans which include, among other things, an integrated and co-ordinated attack from the air, sea and land”.
It follows stern warnings from Israeli leaders, including Mr Netanyahu, who said on Wednesday that “every Hamas member is a dead man”.
Israel's Minister of Defence Yoav Gallant also said the army would “wipe Hamas from the Earth”.
Israel on Friday said that all Palestinians in Gaza – one of the most densely populated areas in the world – must “evacuate”, from the northern part of the enclave within 24 hours, effectively giving 1.1 million civilians few options because parts of southern Gaza are open farmland, or small communities with no hope of absorbing the displaced.
In separate calls with Mr Netanyahu and Palestinian President Mahmoud Abbas, US President Joe Biden stressed the need for civilians to be protected.
Mr Biden reiterated US's “unwavering” support for Israel in his call with Mr Netanyahu, the White House said. Mr Biden also “affirmed his support for all efforts to protect civilians”.
In his call with Mr Abbas, Mr Biden said the deadly Hamas attack was not representative of Palestinians' right to “dignity and self-determination”, the White House said.
Mr Biden also detailed the need to not widen the scope of conflict in the region in the two calls.
A blockade of Gaza since Hamas took over in 2007 – which has involved banning the entry of so-called “dual use” items that the Israelis say could be used for war – has had a crushing impact on the civilian economy.
But since the October 7 attack, Israel has also cut off electricity and water supplies and the halted the entry of all items including food, fuel and medical supplies, leading to power shutdown and a humanitarian crisis condemned by the UN as “collective punishment”.