Hamas says 13 hostages killed in Israeli air strikes on besieged Gaza

At least 13 Israeli and foreign hostages held in northern Gaza have been killed in Israeli air strikes in the past 24 hours, Hamas's armed wing has said.

In a surprise assault early Saturday, Hamas fighters stormed Israeli communities near the Gaza border and fired barrages of rockets, in an attack that claimed more than 1,200 lives.

Israel says Hamas has taken more than 150 people hostage, including both civilians and security forces.

"Thirteen prisoners... including foreigners" were killed in five locations targeted by Israeli fighter jets, the Qassam Brigades said in a statement.

Israel has rained air and artillery strikes on blockaded Gaza — a densely populated enclave of 2.4 million people — flattening buildings and killing more than 1,500 people.

According to the Hamas media office in Gaza, at least 500 children are among the dead.

The Qassam Brigades had warned this week that "every targeting of our people without warning will be met with the execution of one of the civilian hostages".

Earlier, Hamas rejected an Israeli order for 1.1 million residents to relocate from northern Gaza ahead of an expected ground incursion into the overcrowded Palestinian territory.

"Our Palestinian people reject the threat of the occupation (Israeli) leaders and its call for them to leave their homes and flee from them to the south or Egypt," the group said in a statement.

"We are steadfast on our land and in our homes and our cities. There will be no displacement," it said.

The United Nations said an order by Israel to flee within 24 hours risked a "calamitous situation".