Israel has systematically obstructed food entering Gaza, aid agencies say, as a UN-backed body confirms a famine.

The United Nations officially declared a famine in Gaza on Friday, blaming "systematic obstruction of aid" by Israel, hours after Defence Minister Israel Katz threatened to destroy the territory's largest city.

An announcement of famine — as has now happened regarding Gaza — is a complicated decision. Here's what must be considered before such a declaration is made.