More than 70 rights groups asked Facebook on Monday to clarify its policies for removing content, especially at the behest of governments, alleging the company has repeatedly censored postings that document human rights violations.

In a letter sent to Facebook chief executive Mark Zuckerberg, the organizations criticize the social media company for cases in recent months where it has deleted content involving police violence, removed iconic imagery from the Vietnam War and briefly suspended accounts belonging to two Palestinian journalists.

"News is not just getting shared on Facebook: it's getting broken there," read the letter, whose signatories include the American Civil Liberties Union, Sierra Club, Center for Media Justice and SumOfUs.