Hamas gunmen killed around 250 people who attended an outdoor music festival in an Israeli community near Gaza at the weekend, a volunteer who helped collect the bodies said on Monday.

"In the area where the party took place, and at the party itself" it could be estimated that "there were 200-250 bodies," said Moti Bukjin, a spokesman for the humanitarian NGO Zaka, based on the number of trucks that ferried away the corpses.

At least 700 people were killed in southern Israel when Hamas forces stormed across the border, shooting people in the communities and towns near Gaza before security forces began fighting back.

"I've been a volunteer at Zaka for 28 years" and after working at a deadly stampede in Meron during a religious festival two years ago, "I thought I reached my end," Bukjin said.

"I wanted to retire after seeing 45 bodies in one place, I thought it was the end of the world," he added.

"Turns out things can be much, much worse," he told AFP in a phone call, as he prepared to return to the south to continue the work of his group, a religious NGO that specialises in collecting bodies in accordance with Jewish law.

"They butchered people in cold blood in an inconceivable way," he said of what he saw near Kibbutz Reim, where the party took place overnight Saturday.