Israeli airstrikes killed at least 21 people in Gaza on Tuesday, local medics said and fighting ramped up, as the Israeli military said it had been targeting command centres used by its Islamist militant foe Hamas.

Palestinians said the school was sheltering displaced people but the Israeli army said it was being used as a command center by Hamas.

The latest in a series of strikes on schools being used as shelters killed three in Nuseirat, according to the Palestinian Civil Defense. Israel has said the buildings were being used by Hamas militants as a command center.