The UN children's agency UNICEF said there was a risk that the number of child deaths directly from bombardment could be eclipsed.

"Our gravest fears about the reported numbers of children killed becoming dozens, then hundreds, and ultimately thousands were realised in just a fortnight," UNICEF spokesman James Elder said.

"The numbers are appalling; reportedly more than 3,450 children killed; staggeringly this rises significantly every day.

"Gaza has become a graveyard for thousands of children. It's a living hell for everyone else."

"There are certainly children who are dying who have been impacted by the bombardment but should have had their lives saved," Elder told reporters in Geneva, via video-link.

Without greater humanitarian access into Gaza, "the deaths from the attacks, they could absolutely be the tip of the iceberg".

He said the more than one million children living in the Gaza Strip were also suffering from a lack of clean water.

UNICEF is calling for an immediate humanitarian ceasefire, with all access crossings into Gaza opened for the safe, sustained and unimpeded access of humanitarian aid, including water, food, medical supplies, and fuel.