The Israeli military says it has launched the first stages of a new major offensive in Gaza, in a development that comes on the same day that US President Donald Trump concluded his visit to the region without securing a ceasefire deal.

The withering Israeli aerial campaign appeared to presage the ground operation Prime Minister Benjamin Netanyahu has vowed to launch if no ceasefire and hostage deal were struck by the time Trump returned to the U.

Israeli strikes on Gaza have killed more than 250 people since Thursday morning, local health authorities said on Friday, one of the deadliest phases of bombardment since a truce collapsed in March, with a new ground offensive expected soon.