International law does not permit the kind of mass evacuation demanded by Israel in the Gaza Strip, Spanish Prime Minister Pedro Sanchez warned on Saturday after Israel told residents in the north of the Palestinian enclave to flee.

"Israel has the right to defend itself but always within the limits of international humanitarian law, which does not authorize the evacuation of Palestinians from Gaza, as the United Nations says," Sanchez said at a meeting of his Socialist Party in the southwestern city of Merida.

"We condemn, completely and without any hesitation, the Hamas terrorist attack in Israel, and also the deaths of Israelis, and we demand the urgent release of all Israeli hostages and captives," Sanchez had previously said as Israel steps up its response to last week's bloody incursion by Hamas which left some 1,300 dead.

A massive retaliatory bombing campaign targeting the Islamist group has killed more than 2,200 people in Gaza.

Sanchez lamented that the latest violence has caused "such anxiety and instability in the region and in the world," and urged a solution involving the creation of a Palestinian state.

Sanchez heads up a coalition government within which leftist allies of his party have strongly criticized the scale of Tel Aviv's response to the Hamas attack.

Israel earlier warned Gaza residents to evacuate the northern Gaza Strip before an expected ground offensive a week on from the deadliest attack in Israeli history.