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Israel's Zaka search and rescue agency on Sunday confirmed the deaths to state broadcaster Kan.
"We collected over 250 bodies from the nature party. They haven't all been collected yet," Zaka said. It later revised the figure to 260.
On a Facebook page set up to help to trace survivors, dozens of photos of smiling young people from Israel and other countries were shared, along with their mobile phone numbers and appeals for information.
While the event was promoted as a peace festival celebrating unity and harmony, it took place on illegally occupied land, close to the Gaza border.
The festival included several artists from across the globe, alongside a line-up of local DJs.
Israeli newspaper Yedioth Ahronoth estimated that 70 per cent of the participants were from Israel, while the remaining 30 per cent travelled from other countries, including the US and Australia.