Israeli strikes have killed more than a dozen people in the Gaza Strip as Israel sends more ground troops into the Palestinian territory to ramp up its offensive against Hamas.

Israel said it had scaled up a ground offensive in northern Gaza as part of an operation to extend so-called "security zones" that provide a protective buffer along the border with Israel.

Authorities in Hamas-ruled Gaza accuse Israeli forces of a "heinous massacre" with a strike on a school they say killed almost 30 people, many of them children.