Robert Piper, the UN's top humanitarian official in the Palestinian territories, told AFP in an interview to mark a new report on living conditions in Gaza all the "indicators are going in the wrong direction".
"We predicted some years ago that Gaza would fast become unlivable on a host of indicators and that deadline is actually approaching even faster than we predicted -- from health access, to energy to water," he said.
A 2012 UN report predicted the Palestinian enclave would be "unlivable" by 2020 if nothing was done to ease the blockade.
In such circumstances "for most of us that unlivability point has already been passed", he said.
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"And yet somehow the Gazans soldier on."
The Islamist Palestinian movement Hamas seized Gaza from secular Palestinian president Mahmud Abbas's forces in June 2007, leading Israel to impose a crippling blockade which critics say punishes all of the two million residents indiscriminately.
Since 2013 Egypt, the only other country with which Gaza shares a border, has largely closed off its crossing and destroyed hundreds of smuggling tunnels that provided a vital lifeline for the economy while also allegedly being used by Hamas to bring in weapons.
Hamas has fought three wars with Israel since 2008, the most recent in 2014.
Tiny Gaza is wedged between Israel, Egypt and the eastern Mediterranean and is barely 40 kilometres long and 10 kilometres wide.
With two million people it is one of the most densely populated places in the world, and has suffered heavily in each of the conflicts.