As Palestinians mourned the deaths of about 30 people over the last two weeks, shot by Israeli forces during protests along the border with Gaza, grainy video footage surfaced Monday appearing to show Israeli troops shooting a Palestinian man across the border fence at a time when he posed no obvious threat — and then rejoicing.

In the video, the man is standing motionless, just inside Gaza. An Israeli sniper is told to take him down. A shot rings out and he falls to the ground.

Another soldier who appears to be videoing the scene whoops with excitement.

“Wow, what a video! Yay!” he exclaims, adding an expletive. “What a video legend.”

After the footage, from an unknown source, was widely broadcast by Israeli media Monday night, the Israeli military said the event it depicted apparently occurred a number of months ago and would now be thoroughly investigated.

But it emerged at a fraught time, after the flare-up along the border over the last two Fridays also left hundreds of Palestinians with gunshot wounds, according to Palestinian medical officials.

The Israeli military contends that it has fired judiciously and precisely to prevent damage to the border fence or harm to its soldiers as protesters, spurred on by Hamas, the Islamic militant group that controls Gaza, have hurled stones and firebombs and rolled burning tyres.

Israel says it also acted to prevent a mass breach of the border that would have led to greater bloodshed and to block Hamas, which Israel, like much of the Western world, classifies as a terrorist group, from using the protests as a cover for armed attacks.

The latest footage, however, comes after a series of earlier videos from the Gaza side appeared to show unarmed protesters being shot, and after a Gazan journalist wearing a protective vest clearly marked with a “PRESS” sign was killed Friday.

Even if months old, the latest video — shot, unusually, from the Israeli side — appeared to undermine Israel’s arguments as it battles international censure and accusations of employing disproportionate force and to provide critics of its policies with additional grist.

“Incidents such as the one in the video published today occurred hundreds of times over the past few weeks in the Gaza Strip, causing death and injuries — with the full support of policymakers and top military officials,” Btselem, an Israeli human rights organisation, said in a statement. “Btselem is deeply sorrowed by the manifestly illegal commands ordering soldiers to shoot at people who pose no threat.”

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Breaking the Silence, an Israeli veterans organisation that opposes Israel’s policies toward the Palestinians, wrote on Twitter, “This isn’t from a few months ago, it’s been 51 years,” referring to the 1967 war when Israel captured the Gaza Strip from Egypt. Israel unilaterally withdrew its troops and settlers from Gaza in 2005, but together with Egypt it has imposed a blockade on the isolated coastal territory, citing security grounds.

Also Monday, the Norwegian Refugee Council, a nongovernmental organisation, provided some of the last footage taken by Yasser Murtaja, 30, the Gazan journalist who was killed Friday. Mr Murtaja, a photographer with the Gaza-based Ain Media agency, had been covering the border protests against the blockade and for the return to lands in what is now Israel. He was supposed to start documenting the struggle of Palestinian refugees in Gaza for the Norwegian Refugee Council.

The footage, some of it shot by a drone, shows protesters praying, burning tyres and facing off against the Israeli soldiers. Mr Murtaja shot video of injured men being rushed away on stretchers and in ambulances and being treated in a makeshift field hospital.

The New York Times