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.

Israeli airstrikes killed at least 100 Palestinians across the Gaza Strip on Thursday, including 27 or more sheltering at a school, according to Palestinian medical authorities, in a stepped-up offensive that Israel's military said is intended to pressure Hamas and eventually expel the militant group.

Israeli strikes have killed more than a dozen people in the Gaza Strip, as Israel sent more ground troops into the Palestinian territory to ramp up its offensive against Hamas. At least 17 people, some from the same family, were killed after an air strike hit the southern city of Khan Younis, according to hospital staff.