Israeli airstrikes and artillery shelling killed 21 Palestinians across the Gaza Strip Wednesday morning as the military offensive entered its sixteenth day.

More than 640 Palestinians, the vast majority of whom have been civilians, including more than 160 children -- have been killed in the assault so far, while Israel has suffered 29 deaths, 27 of whom have been soldiers.

The Gaza Strip continued to be bombarded from air, land, and sea on Wednesday morning, a day after the the United Nations' Palestinian refugee agency UNRWA accused Israeli of shelling a UN school sheltering the displaced for the second time in two days.

"UNRWA condemns in the strongest possible terms the shelling of one of its schools in the central area of Gaza," it said in a statement.