As the Israeli siege of Gaza continues, the area is "on the edge of catastrophe", a BBC reporter on the ground says.
In Khan Younis in the south, hundreds of thousands of people have arrived, after Israel warned them to leave the north.
People are sleeping in the streets, running water is limited, and there are queues for bread outside bakeries.
Israel has blocked supplies of food, water, fuel, and electricity since the deadly Hamas attacks on 7 October.
But the Israeli military denies there's a humanitarian crisis, insisting there are supplies of water and electricity.
US President Joe Biden will visit Israel on Wednesday to hear about its plans for a ground attack in Gaza.
A 13-year-old British girl who went missing when Hamas attacked Kibbutz Be'eri is now confirmed to have been killed.
More now from the UN agency for Palestinian refugees, which has been highlighting the lack of clean water in the Gaza Strip and warning that waterborne diseases are going to start spreading.
"Most of Gaza, in fact the vast majority, does not have running water," Juliette Touma, from the UNRWA, told Radio 4's Today programme.
"This is why its is absolutely fundamental that we are allowed to bring in fuel into the Gaza strip for the pumping stations across the strip so that clean water resumes across Gaza."
Gaza’s last functioning seawater desalination plant - which makes seawater drinkable - shut down yesterday, the UNRWA said.
It also said that one line of water was opened yesterday for three hours in Khan Younis in Gaza's south. But in that time, only 14% of people in Gaza received water, the organisation said and reiterated its warning that without clean water, people will die.
