Two Canadian schoolgirls, aged 16 and 17, have been charged with second-degree murder after footage of them apparently beating a 19-year-old girl to death surfaced on Facebook.

The footage was quickly removed from the site but is still available online. Serena McKay can be heard saying "sorry" to the girls as they kick and punch her bloodied face and say they are going to kill her.

McKay was found dead near her family home on 23 April. Hundreds of people attended a memorial event four days later.

The two girls have not been named but with Serena they attended Sagkeeng Anicinabe High School on the First Nation reserve in Manitoba, 100 km north of Winnipeg.

Sagkeeng Anicinabe High School principal Claude Guimond said drugs appear to be involved: "After seeing what I saw on the video, you know what? There's nobody in their right mind (who) would do something like that, unless they were extremely high on whatever and just totally, like, out of it," he told CBC.

A Facebook spokesperson told CBC the video appeared to have been removed. "This was a horrific tragedy and our hearts go out to the family and friends of Ms. McKay," said Facebook.

"We have not been able to locate the video on Facebook, and are working with law enforcement as they investigate."

Facebook Live under the spotlight

Facebook has been forced to react to criticism of the way it monitors its live streaming service recently after a spate of deadly incidents were broadcast live. These included the murder of 74-year-old Robert Godwin by Steve Stephens in Cleveland, the suicide of James Jeffrey in Alabama, and the murder of an 11-month-old baby by her own father in Thailand.

Facebook founder Mark Zuckerberg has responded to the spate of deaths by announcing the recruitment of an additional 3,000 monitors to review videos reported as being graphic, disturbing or inappropriate.