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.

The air and artillery strikes were focused on the northern section of the tiny, crowded enclave, where dozens of people including women and children were killed overnight, said Gaza Health Ministry spokesman Khalil al-Deqran.