At least 20 people have been killed in a crush at an aid distribution centre in southern Gaza run by the US- and Israel-backed Gaza Humanitarian Foundation (GHF), the organisation and a local hospital say.

The GHF said 19 were trampled to death and one was stabbed "amid a chaotic and dangerous surge" at its site in the Khan Younis area. It added that it believed people "armed and affiliated with Hamas" fomented unrest.

But Gaza's Hamas-run Government Media Office denied the claim and accused the GHF of trying to "cover up" a crime.

Nasser hospital in Khan Younis said it had received the bodies of 21 people who died from suffocation as a result of tear gas inhalation and a crush at the aid site.

It is the first time the GHF has confirmed deaths at one of its aid sites.

The Government Media Office accused the US private security contractors, who distribute aid for the GHF, of causing the crush by closing the gates of the site after thousands of people had gathered in narrow channels to collect food, and then firing canisters of tear gas and live rounds towards them.

In a graphic video shared on social media and verified by the BBC, a witness standing on a cart filled with the bodies of six boys and men at Nasser hospital said they had been crushed between fences set up at the GHF site while waiting for food handouts.

"They are children. What is it their fault dying for aid?" the man shouts as he holds up the body of one of the boys.

"What happened is [that] at the door of the aid [site], the foreigners made a fence here and a fence here," he gestured. "The boys went to the front and the people came and stepped on them."

There have been almost daily reports of Palestinians being killed while seeking aid since the GHF began operations in late May. Witnesses say most have been shot by Israeli forces.

The UN human rights office said on Tuesday that it had so far recorded 674 killings in the vicinity of the GHF's four sites in southern and central Gaza over the past six weeks. Another 201 killings had been recorded along routes of UN and other aid convoys, it added.

Before Wednesday, the GHF had denied that there had been any deadly incidents in close proximity to its sites and accused the UN of using "false and misleading" figures from Gaza's Hamas-run health ministry.

The Israeli military said last week that it recognised there had been incidents in which civilians had been harmed and that it was working to minimise "possible friction between the population and the [Israeli] forces as much as possible".

The GHF uses private security contractors to distribute aid from sites in Israeli military zones.

The UN refuses to co-operate with it, describing its set up as unethical.