Humanitarian aid must be allowed into Gaza, where life is becoming "impossible" for the population, the Doctors Without Borders (MSF) charity urged on Friday.
The organisation's president for France, Isabelle Defourny, told AFP on her return from southern Gaza that aid deliveries needed to be "sufficient to address the emergencies" suffered by the civilian population there.
"We have said again and again that the Gaza Strip has become uninhabitable, but now it's actually becoming impossible to live there," she said.
More than two million people were living "virtually outdoors", she said, with only plastic sheets for cover.
"As cold weather approaches, this is going to go very badly," she said, adding that current humanitarian aid had been "in no way sufficient".
The comments came ahead of the first anniversary of Israel's war on Gaza.
Ongoing Israeli bombardments and the continuing destruction of infrastructure had left formerly busy transport routes in ruins, Defourny said.
MSF called on Israel to reopen the Rafah border crossing between Gaza and Egypt to allow aid and basic foodstuffs to get through, she said.
Cogat, an Israeli government agency, said this week that aid was getting into Gaza continuously, and more than a million tonnes had been delivered in total since the start of the war, but Defourny said this was still insufficient.
On Thursday, MSF had already reiterated its call for Israel to open "vital land borders" with Gaza.
In a statement, it also called for a "sustained ceasefire", and for an immediate end "to the mass killing of civilians".