The calls from HRW and other rights groups come as websites like Facebook and YouTube delete content deemed "inciting or promoting violence".
But the purge also risks destroying crucial evidence that could prove be used in human rights abuse cases.
"Some of the content that Facebook, YouTube, and other platforms are taking down has crucial and irreplaceable value as evidence of human rights atrocities," said Belkis Wille, senior crisis and conflict researcher at Human Rights Watch.
"With prosecutors, researchers, and journalists increasingly relying on photographs and videos posted publicly on social media, these platforms should be doing more to ensure that they can get access to potential evidence of serious crimes."
The 42-page report, called "Video Unavailable: Social Media Platform Remove Evidence of War Crimes", advices social media platforms to "develop an independent mechanism to preserve potential evidence of serious crimes".
"Independent civil society organisations and journalists have played a vital role in documenting atrocities in Iraq, Myanmar, Syria, Yemen, Sudan, the United States, and elsewhere – often when there were no judicial actors conducting investigations," it reads.
War crimesAt least ten cases across Europe - including in Germany, Finland, the Netherlands, and Sweden - secured convictions against individuals linked to war crimes in Iraq and Syria.
Their cases involved videos and photos shared over social media.
The International Criminal Court (ICC) used content found on social media in proceedings, while activists have used the platforms to compile evidence of regime chemical weapons attacks in Syria and security force violence in Sudan.
In one example, in 2016, Swedish authorities investigated Haisam Omar Sakhanh, a Syrian man who was seeking asylum there for lying about a former arrest in Italy on his asylum application.
An investigation uncovered a video published three years earlier which showed a Syrian non-state armed group extrajudicially executing seven captured Syrian government soldiers in Idlib.
Sakhanh was seen participating in the executions, and a Swedish court convicted him of war crimes and sentenced him to life in prison.
In a 2019 Dutch case, a man was convicted of a war crime for posing next to a corpse on a crucifix in Syria and then posting the photograph on Facebook.
After reviewing Facebook, Twitter, and YouTube content that Human Rights Watch has cited in its reports to support allegations of abuse since 2007, HRW found that from 5,396 total pieces of content referenced in 4,739 reports - the vast majority of which were published in the last five years - it found that 619 (or 11 percent) had been removed.
A European law enforcement officer investigating war crimes told Human Rights Watch that "content being taken down has become a daily part of my work experience. I am constantly being confronted with possible crucial evidence that is not accessible to me anymore".
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